Obviously, McCullough is bound in large part by the actual, historical Marcus Livius Drusus and what he did. Nonetheless, I assume she exercises a considerable degree of creativity as well, and I find the arguments she gives him for extending citizenship to Italians to be unconvincing.
Admittedly, Drusus arguing for Italian citizenship and McCullough portraying his arguments have a serious problem. Persuading people to extend their rights and privileges to people below them is hard. It has the effect, after all, of diluting their power. Why would people want their power diluted? So what argument is convincing? It is very hard to tell, but the argument McCullough has Drusus rely on would definitely not be it.
Consider. At one point in the story, Drusus' Italian friend warns him of a plot to assassinate the consuls. Drusus, whose championship of the Italians emphatically does not extend to condoning terrorism, duly reports the plot to the more sympathetic consul. The consul's immediate impulse is to avoid the planned ambush. Drusus urges him, instead, to walk into it, but with a secret armed guard so the plot can be defeated and the plotters executed. But why does he want to see the plot attempted and thwarted rather than quietly avoided? To warn the Italians that the Romans mean business and that violence will get them nowhere? No, to warn the Romans that the Italians mean business and will resort to violence if not given citizenship. If I were a Roman, this would not make me want to extend citizenship to the Italians. It would make me want to take a hard line against them. Drusus constantly warns that to deny the Italians citizenship will lead to war. He turns out to be right. But again, this is a terrible argument to make.
Some points of comparison are in order. Although Drusus implemented something very much like the program of the Gracchi, McCullough portrays him as a conservative reformer along the lines of Bismark or Disraeli -- the kind who reform the system to preserve it because they know that there is nothing conservative about letting the system tear itself apart. The difference is not so much in the program as in how one sells it.
At the end of First Man in Rome, and several years before the events of The Grass Crown, there was a serious grain shortage. Rome's poorest citizens were facing the real prospect of starvation, and less-poor citizens were facing a serious squeeze. The demagogue Saturninus took advantage of the situation and made a lot of promises about cheap grain that he could not keep because there simply was no grain to be found. Soon, Rome was facing the prospect of revolt. The revolt was put down and the day was saved by the last-minute arrival of new grain shipments, but not until Rome's ruling class got a good scare. For Drusus to argue a few years later that this showed that Rome really needed a better system of famine relief would be eminently reasonable. To argue that the government should always guarantee a price of grain only slightly above wholesale in good times, that it should have a regular fund dedicated to this purpose, that the system should be up and running and everyone should know about it and be convinced that it would work before the next grain shortage can be a perfectly sensible, even conservative argument.* Drusus could point out that if such a system is in place by the next grain shortage, it will keep a demagogue like Saturninus from taking advantage of the situation and allow Rome to get through it with scarcely a ripple. Rome's conservative politicians might very well be convinced. It would be another thing altogether to make this argument in the midst of a food shortage, with Saturninus and his crowd rioting in the Forum. Aside from the problems of setting up such a system in the midst of a crisis, to do so would look very much like caving in to blackmail, and that prospect would make most people want to resist.
Or consider the matter of Marius' veterans. They were drawn form Rome's poorest citizens, who many doubted would be up to the job. But the time of The Grass Crown, he had proven their effectiveness as soldiers and been retired with grants of land a nice, safe distance from Italy. It would be eminently reasonable, even conservative for Drusus to point out that, while Rome would no doubt have to rely on such men in the future, not to make provisions for their retirement would be to unleash on Rome a bunch of men with minimal civilian employment history, no property, no future and no hope, but lots of military experience and training, and if that doesn't scare the hell out of you, think it over some more.** It would be rather different, though, if such soldiers were about to demobilize, or already had demobilized and were stirring up trouble. Once again, it would look like submitting to blackmail, and nobody wants to submit to blackmail.
The same applies to Drusus' arguments that if we don't extend citizenship to the Italians, they will wage war on us. That is, quite simply, a threat, and no one likes being threatened. Besides, do we actually want to extend citizenship to people preparing to go to war with us? That sounds like a terrible idea to me. It makes Drusus sound like a coward urging surrender and submission in the face of threats. That is terrible politics!
*That leaves open the question of how to actually have grain available if there is none to be had. Presumably the government would buy it up in good times and store it for sale in bad times, but Drusus never actually says so. He talks more about the funding mechanism and how to get the actual grain itself.
**Once again, in McCullough's book, Drusus appears to endorse regularly using poor men as soldiers and retiring them with grants of land a nice, safe distance away. But he does not introduce actual legislation to that effect. Whether the real Drusus weighed in on the issue I do not know.