While it seems a near-certainty that the Trump Administration will be extraordinarily corrupt and incompetent, and while Trump himself give all evidence of being undisciplined, impulsive and capricious, utterly uninterested in the responsibilities of power and the tedious details inherent in the presidency, he will doubtless be a strong leader in at least one regard -- maintaining party discipline.
Traditionally, European political parties have been ideological parties, with strong party discipline, while American party have been diffuse coalitions and rather loosely bound. European politics has taken place almost entirely through parties, to the extent that disagreement with one's party means the end of a political career. Party discipline in U.S. politics has been more difficult to maintain -- at least during the 20th century.
Parties were stronger in the 19th century, though not always better-smelling. It may seem almost blasphemous to compare Donald Trump to Thomas Jefferson, but as party-men there are actually parallels. As President, Jefferson's legal powers were strictly limited. Congress could pass unfriendly legislation and override his veto, the federal courts could declare his actions unconstitutional, states could decline to enforce federal actions and pass unfriendly legislation of their own,* and private citizens could mock him. He had no authority to command any of these, only to make suggestions that they were free to disregard. He could only issue orders and count on having them obeyed within the executive branch of the Federal Government, which was much smaller than than it is now, and even within the executive branch, his powers were limited to what laws passed by Congress authorized him to do. (Except in foreign policy, where he had greater leeway).
Yet if his legal authority as President was strictly limited, his moral authority as leader of his party was immense. Jefferson had no authority to command Congress, or state governments, yet as leader of the Republican Party,** any suggestion he made would be instantly followed by Republicans everywhere. He had no legal authority to compel such obedience; his authority rested entirely on Republicans' confidence in him, his wisdom, and his ideological purity. This authority existed entirely within the confines of the Republican Party. He had no influence whatever with Federalists; if anything, the mere thought that Jefferson was in favor of something was enough to convince Federalists to reject it.
When Jefferson stepped down, no other leader emerged with his moral authority. Instead, parties enforced discipline by patronage. Presidents had immense authority appoint federal officials and could staff the federal bureaucracy almost from top to bottom. Of course, they could not possibly know who was the best choice for postmaster for every post office across the country and the like. So it was the clear understanding that the President would follow the advice of every member of the President's party in the President's good graces. And since their ability to dispense patronage and federal jobs was a major source of Congressmen's power, this gave them a strong incentive to stay in the President's good graces. If they quarreled with the President on a matter of legislation, the President was apt to stop listening to their advice on matters of patronage. If the rebellion was serious, the President might even start taking advice from a political rival instead to build up a rival faction to displace the rebel. (This was what James Buchanan did when Stephen Douglas rebelled against him in the matter of admitting Kansas as a slave state against the wishes of the inhabitants. It didn't work).
Not, however, that, as with Jefferson's moral authority, patronage applied only to member's of one's own party. Members of the party opposing the President were shut out of the spoils altogether. They expected nothing of the President and received nothing. Their hope lay, instead, in having one of their own elected President and getting patronage from him instead. The movie Lincoln is instructive. When Lincoln uses patronage to bribe Democrats into voting for the Thirteenth Amendment, what is scandalous is not that he is corralling votes by patronage, but that he seeking votes from and extending patronage to the opposing party. That, and not the use of semi-bribes, was what would have shocked his contemporaries.
The movie also shows the power of moral authority within a party. Preston Blair holds no office whatever, has nothing to promise and no credible threats. Yet all the conservative wing of the Republican Party vote yes or no on the Thirteenth Amendment based on a mere hand gesture from him. (Thaddeus Stevens holds similar authority with the Radical caucus, although, of course, the Radicals need much less persuading).
With the end of the Spoils System, enforcing party discipline became increasingly difficulty until relatively recently, when a new method was discovered -- the primary challenge. This one actually appears to have started with the Democrats, when they primaried Joe Lieberman for supporting the Iraq War. But, of course, the Republicans are the ones who have perfected it. Every Republican now lives in terror of being found ideologically impure and defeated by a primary challenge. But who determines who is and is not ideologically pure? Until recently the answer was a little unclear. Was it Fox News? The most ideological Republicans in office? Or was the entire party inherently impure because they could neither oust Obama nor sufficiently humiliate him?
But now that there is a Republican President, we have an arbiter of ideological purity. Still, up till now Presidents have been reluctant to proclaim members of their party impure and unleash a primary challenge on then. But in Donald Trump, we have a leader who has no such qualms. And he maintains party discipline with a whole new method never imagined before -- his Twitter account. With a single tweet he can tank stocks and reduce defiant Republicans to submission. Kevin Drum mocked timid Republicans for being frightened into submission by the "nasty phone calls, letters and tweets" that follow a hostile Trump comment and called on politicians to have thicker skins. But the comments section responded by pointing out what is really intimidating them -- a hostile tweet is a call for a primary challenge. We haven't reached primary season yet, but when it arrives presumably Trump will raise a primary challenge to any Republican by a single tweet, and pick winners and losers in the primary the same way.
And outrageous as it may seem to compare Donald Trump to Thomas Jefferson, who controlled his party by pure moral authority, the fact is that Trump tweets have such power because he has real moral authority among Republican primary voters, who hang on his every word as arbiter of the true party line. So long as he retains such powerful authority over the minds of the Republican base, it appears that Trump can control the Republican Party with tweets alone.
*The Constitution is clear that in case of a conflict between federal and state law, the federal law will prevail. However, if the federal government attempts to commandeer states to enforce its laws, states are free to decline, and if a state passes legislation that the President considers unconstitutional, his only recourse is to direct the Attorney General to bring a suit to have it declared unconstitutional and allow it to play its way out through the federal courts.
**This was not the modern-day Republican Party. It was the ancestor of the Democratic Party. In Jackson's time the Republican Party split into the National Republicans, which later changed their name to Whigs, and the Democratic Republicans, which later changed their name to Democrats. The Whig Party eventually fell apart over the issue of Slavery and a new Republican Party, named after Jefferson's party, was formed. Needless to say, it was limited to free states and had no presence in the South.