Another way of looking at it is, what do Republicans fear if Trump wins. Presumably they are attempting to achieve the opposite of what they fear. And then what the effect of a brokered convention would be.
Republicans fear a landslide defeat in the Presidential election. In that case, what the Republican establishment would want would be to win the general election for President, and they see Trump as an obstacle. And it is true, current polls show Hillary ahead by roughly ten percentage points. But then again, most of those polls show Hillary with only around 50% of the vote, and a large number of undecides. Presumably some of those will break for Trump. The polls are from March. The election is in November. A lot can change in that time.
But assuming the results head steady and Trump meets a crushing defeat, does the Republican establishment think they are more likely to win if they hold a brokered convention and deny Trump the nomination? Consider who they will be dealing with. There are hardcore Democratic voters who would never vote for a Republican anyhow. Those will not be swayed. There are hardcore Republican voters who will vote for the Republican candidate, no matter what. There are hardcore anti-Trump Republicans who will vote for a normal Republican, but would rather stay home or even vote for Hillary than vote for Trump. These actually will be persuaded. But how many of them do the Republicans think there are? Certainly not all Republican voters, since many voted for Trump. And from the Republican voters who voted for someone other than Trump, you have to subtract the hardcore Republicans who will vote for their party's candidate regardless. So this is a voting bloc, but we can seriously question how large. Against these are Trump supporters. All Trump supporters will be furious at the party establishment for its betrayal in denying them the nominee of their choice. Yes, some are partisan Republicans who will vote for their party regardless, but a whole lot will react with fury against the party that cheated them of their choice. Some may simply stay home, but a whole lot are apt to support Trump in a third-party bid or as a write-in. The number of Trump supporters enraged enough to refuse to vote for the brokered convention nominee will certainly be significantly greater than the number of anti-Trump Republicans who would otherwise sit out. And then there are the undecided voters. But as discussed before, the sight of a party committing ceremonial self-disembowelment usually convinces undecided voters that they do not want to entrust their country to the circular firing squad.
On the other hand, suppose the Republicans do lose the presidential election? So what? They have lost presidential elections before. Indeed, losing an election is a routine matter. It is not any party's preferred outcome, obviously, but hardly an unprecedented catastrophe. Go down, brush the dust off, then get up and fight the next election. It happens all the time.
The Republicans are afraid of losing in down-ballot elections. In that case, they would be writing off the presidency as hopeless, but trying to save as many other offices as possible. Assuming Trump loses by a landslide, would he have negative coattails? Would he induce others who would have voted Republican in down-ballot races to either stay home or vote Democratic? At least some have suggested so. I do not pretend to have investigated the issue enough to have a reasonable opinion. But even if Trump does endanger Republicans in down-ballot elections, the same basic calculus applies as to the presidency. If Trump causes the Republicans to lose many down-ballot elections they would otherwise have won, then the results will be anomalous and short-lived. It will be rather like the 2008 election. The combination of a failed war and an economic crisis caused Republicans to lose a large number of down-ballot elections in jurisdictions they normally would have won. They reclaimed those offices in the next off-election of 2010, and added spectacularly in 2014. Any normally Republican Congressional district, governorships, state legislatures, etc. that may be lost as a result of revulsion against Trump will be short-term losses easily regained next time around. By contrast. if the Republican establishment violates all accepted political norms to deny the nomination to Trump after he wins a plurality of delegates, they face the real danger, not just of a third party challenge by Trump this year, but of Trump followers permanently bolting the party and forming a third party. A true third party could not possibly form between the nominating convention and the general election, but two years would be sufficient time for large portions of the Republican rank-and-file, feeling utterly betrayed by their leaders (and with some justification) to establish a real third party, perhaps drawing off disaffected Democrats as well, and troubling the Republicans (and perhaps the Democrats) for some time to come.
Republicans are afraid of what the Democrats might do with the triple crown. This at least begins to make some degree of sense. If there is anything the last twenty-odd years have proven, it is that electoral losses are fleeting, and that disaster in one election tells a party nothing about what will happen next time around. But policy changes are not so easily gotten rid of. George Bush's war in Iraq may be generally seen as a mistake, but the basic elite consensus that we should respond to any foreign policy crisis with military intervention remains. Bush's increased surveillance have actually grown under his successor. And much as Republicans hate Obamacare, they have no realistic plan for getting rid of it. Republican may fear that Hillary Clinton, with a sufficient Democratic majority in Congress for even two years, may be able to pass measures they consider disastrous but would be unable to undo upon electoral recovery. Obamacare would be an obvious example of what they fear and wish to avoid. In that case, their goal would be to hold onto the House and/or the ability to filibuster in the Senate in order to block any measures Hillary might have in mind.
To this I can only say that I see no evidence that Hillary has any sort of very ambitious agenda, certainly nothing like the improbable promises that Bernie Sanders is making. But let us grant that Republicans seriously over-estimate the radicalism of Democrats and have exaggerated, unrealistic fears of what they will do once in power. Even then, splitting the openly splitting the party risks being just as bad for Republicans' chances in Congress as having Trump as nominee. (See above). And it carries a much greater risk of leading to a third party, which would cripple their chances of stopping the Democrats for a long time to come. If Republicans fear what the Democrats might do with power, they would be much wiser to accept Trump, write off the Presidency (i.e., passively allow him to be defeated by not expending too many resources on his behalf), and focus on protecting vulnerable down-ballot seats.
Republicans want to preserve the ideological purity of their brand. Okay, this at least makes some degree of sense. Libertarians want to preserve their party as the party of minimal government and see Trump as an authoritarian ready to use government for his ends whenever he pleases. Neocons are sincere in their dedication to preserving democracy and home and promoting it (at gunpoint) abroad and see Trump as a bull in a china shop, indifferent to any law or institutions. But there are several problems here. One is that neither libertarians nor neocons are exactly spotless in this regard.
The libertarian wing of the Republican party has fully signed on the government to the extent it means waging war, expanding domestic surveillance, indefinite detention, "coercive interrogation," and any other measure meant to advance the War on Terror. Their claim to be opposed to all "government" is therefore highly questionable. To explain that these are "essential core functions" that they favor, and that they only oppose the New Deal functions of government puts libertarians in an awkward spot indeed, given that there is next to no support for rolling back the New Deal. As for necons, they, too, are tainted with these measures, which calls their support for "freedom" and "democracy" and opposition to "authoritarianism" very much into doubt.
A second objection is (as hinted at), the basic program of the libertarians, to roll back the New Deal, to end Obamacare, turn Medicaid into a block grant as a preliminary to squeezing it to death, to turn Medicare into a voucher system (i.e., Obamacare) with an ultimate goal of phasing it out, to turn Social Security into a 401-k, and to devote the saving to huge income tax cuts, particularly at the top, is wildly unpopular. Nor the the neocon program of starting a war at every possible opportunity have a lot of support with the general public. If this is the ideological purity that Republicans want so much to preserve, then they are truly bent on destroying their party in the name of virtue. If Trump supporters were to start a third party and the Republican elite to openly commit to such a program, then Trump's party would end up transforming from the third party to the second and leave the Republican donors out in the cold.
A third objection is that the ideological difference between Trump and the Republican establishment in practice (if not in theory), is much less than it appears. The two have reached a sort of compromise. Whether or not to roll back the New Deal remains a matter for debate in think tanks that no one seriously dares broach with the general electorate. In the meantime, Trump has adopted the Republican program on wars and starve-the-beast tax cuts, while the Republican establishment is moving toward Trump on immigration and foreign trade. In the words of Jonathan Chait:
The policy content of the primary fight has receded almost entirely. Trump may be more effective than other Republicans at harnessing certain conservative impulses — xenophobia, nationalism — but he barely differs from Ted Cruz in the specific proposals in which he expresses them. Trump has attracted the support of the majority of Republican voters who favor higher taxes on the rich, but Trump himself would reduce them. Trump and Cruz oppose comprehensive immigration reform and have postured as tougher than each other without settling any specific disagreement. Trump attacks free trade more viscerally than other Republicans, but both he and Cruz have the same stance (oppose the Trans-Pacific Partnership, promise to implement some unspecified better deal). Trump is promising to appoint conventional right-wing jurists to the bench. There is no longer any substantive wedge between the GOP and its front-runner.
Instead, Republican-elite loathing for Trump has three sources. First, they recognize that his deep unpopularity among the general public makes him a historically awful nominee. Second, his egomania, lack of interest in policy, and history of off-the-reservation statements and behavior give them justifiable reasons to doubt he will stay committed to their agenda even if somehow elected. And third, they find his persona repellant.It's personal. Maybe Republicans are not afraid of Trump so much as repulsed by him, to the extent of preferring damage to their brand to being associated with such a man. Republicans may not want their party tainted with Trump's ugly brand of xenophobia and demagoguery. In that case, they might be willing to see a third party form, so long as it siphoned off the really ugly elements. The would trust such a party would dwindle away and not cause any long-term damage.
This approach has its problems, though. One is that Republicans have been playing footsie with the xenophobes and demagogues for long enough that siphoning them off might do their brand a lot more long-term damage than they expect. But then again, that is the Democrat in me speaking. Republicans might not agree. The other problem is that even if can successfully purge themselves of Trump's style, they will have to decide what to do with his substance. Do they want to be the party of interventionism or disengagement, of international trade or protectionism, of immigration or anti-immigration? To say nothing of whether they want to continue trying to roll back the New Deal or to make peace with it. Maybe Republicans believe that if they run Trump and lose it will be hard to argue that their secret to success is an ideological alignment in his direction, while dumping Trump and losing will make it easier to adopt a Trumpism without Trump. The problem with this calculation, once again, is that Trumpers may respond to being illegitimately denied the nomination by bolting and forming a third party, a thing that will make ideological realignment by Republicans more difficult rather than less.
Or maybe Republicans just can't stand Trump's vulgar style and obvious status as a liar and conman. That is certainly Chait's opinion. But plenty of Trump supporters have already said that they know he is a liar and a con man, they just think the Republican leadership is equally deceitful, so they prefer a liar and con man who they know shares their values.
Republicans are afraid of what a Trump victory would do to their brand. Maybe Republicans' real fear is not the Trump would lose, but that he would win and make such an utter hash of things that the Republican brand would suffer for another generation. In that case, the Republicans might be willing to write off this one election to keep Trump from damaging their brand any further. The problem with this approach is that it is exchanging a hypothetical future harm for a real present one. Yes, it is certainly true that Trump might win the presidency and make such a hash of things as to damage the Republican brand for a long time. Or he might lose and spare Republicans the headache. Or he might win and have fairly conventional advisers who are able to reign him in. So any harm Trump might do his party in victory remains remote and hypothetical. On the other hand, the harm the Republican establishment would do their party by denying the party faithful their choice of nominee would be immediate and real. See above for what it might be. Ultimately I fully agree with Daniel Larison in explaining why Paul Ryan does not openly come out against Trump:
Ryan is at the height of his career in the House now that he is Speaker, and he presumably has a long career ahead of him in Congress or Wisconsin state politics if he wants it, so gratuitously antagonizing up to half of the Republican primary electorate across the country may not seem like a terribly smart move. . . . He might also reasonably conclude that working to destroy the Republican coalition by openly joining an anti-Trump effort would do more harm to the party in the long term than having Trump as the nominee for a few months. Anti-Trump Republicans want to scuttle the Republican ship to prevent Trump from taking it, and they wonder why Ryan doesn’t share their desire to sink it. I submit that it’s because he is in a leadership position, and he has judged that it is better for the party to suffer Trump briefly and recover than it is to destroy itself to keep it out of Trump’s hands.Which leads to the final possibility.
Republicans are so afraid of a Trump presidency that they are prepared to destroy their party in order to prevent it. In this version, Republicans are not afraid of losing this election, or even of the long-term damage a Trump candidacy may do them. They are afraid of what Trump might do to the country if he wins, so afraid that they are willing to wreck their party to prevent such an outcome. If so, then Republicans are to be commended for their statesmanship. However, such statesmanship is extremely rare, so I think it is most unlikely in this case. Besides, if they really are afraid of what Trump might do in office, there would be more obvious ways of opposing him that would do less long-term damage to their party than a brokered convention, such as openly speaking out about their fears of what a Trump presidency would mean.
In short, no matter what the motives of the Republican establishment, I do not see how their holding a brokered convention can advance them. My conclusion, therefore, is that they will end up holding their noses an acceding to a Trump candidacy. Certainly if I were a Republican consultant, that is what I would recommend.
And finally, a word of advice I would give as a Democratic consultant. Democrats, needless to say, would love to see the Republican Party destroy itself. Apparently one reason Trump was able to rise to the top of the Republican field with such ease was that none of his rivals bothered to do any serious opposition research on him. Democrats, while enjoying the popcorn, have apparently had more advance notice and done some such research. By one estimate, only 20% has surfaced so far. Presumably Democrats are holding their ammunition in reserve until after Trump becomes the nominee. But if the Democrats really want to persuade the Republican Party to start destroying itself, they would be well-advised to start unleashing their material right now. Make his polling numbers sink right now, both the reduce the chances of his getting an absolute majority of delegates and to make him look like a really bad general election candidate. These things will make it more likely that the Republican establishment might try something really stupid to stop him, rather than just write off this election and move on to the next. It couldn't happen to a nicer party.