We know very little about the tribune of 91 (not to be confused with his relative, tribune of 122 and opponent of Gaius Gracchus). His entire career fits into exactly two chapters of Appian, with a further sentence of 1.37 devoted to the Varian law, which has been the subject of debate for a century and a half. Velleius Paterculus likewise only has two chapters in which Drusus features. Florus apparently devoted a lengthy section to him, but his epitome only preserves this discussion as a single chapter. Livy has a brief note in the Periochae on Drusus which tells us pretty much nothing.

It's very hard to tell what the hell was going on with Drusus. Appian describes him as proposing his Italian bill from the beginning, along with proposing the foundation of a number of colonies to secure the support of the urban plebs. Appian says that only then did he propose the transfer of the law courts to the senate from the equites, by attempting to increase the size of the senate from equestrian enrollments. The defining feature of Appian's description of Drusus is one of compromise, which Appian blames for his failure. Drusus, he argues, pissed off the senate by proposing to add equestrian enrollments to their roll, pissed off the equites by suggesting a transfer of the law courts, and pissed off the Italians by proposing the foundation of colonies (despite what wikipedia and the internet will tell you, colonial foundations and land distributions are not the same thing. The former was generally favorable to the urban plebs and the latter to the rural plebs and Italians). Only the urban plebs were satisfied by their part of the deal, says Appian, which I'm afraid I cannot agree with. Appian spends the majority of Book One describing the urban plebs violently resisting any attempt to enfranchise the Italians or even distribute them equally among the voting tribes. Yet he conveniently forgets that conflict here, I think simply because he does not understand what support Drusus could possibly have had. Appian suggests that the Italians killed Drusus, out of fear that his colonial plan might infringe on their right to the ager publicus.

Velleius Paterculus says pretty much the same thing, but he reverses the order. Drusus' main aim, he claims, was the transfer of the law courts to the senate. He proposed a number of bills in favor of the urban plebs, says Velleius, to secure their support ultimately for the transfer of the law courts. But the senate misunderstood the purpose of his popularis legislation and he was forced to propose a bill enfranchising the Italians, which resulted in his murder--the implication in Velleius Paterculus is that Drusus was killed by the senatorial class, although Velleius seems consistently to fail to understand that the urban and rural plebs were decidedly different.

It's hard to see from Florus' description how Velleius could be right, though. Florus agrees that Drusus tried to court both sides (not necessarily unusual for a tribune--the four leges Clodiae of 58 were successful in exactly this aim, and we already see Gaius Gracchus trying to do the same) and essentially was carried away by his popularis program--in promising simultaneously benefits for the urban plebs and Italians he doomed himself to violent measures and attacked the consuls. Florus' testimony is pretty confusing though, and is generally largely discarded--he seems to think, for example, that Drusus died a natural death.

Livy says pretty much the same thing as Velleius Paterculus, that Drusus was an agent of the senate's desire to transfer the law courts from the equites and tried to rally the urban plebs with spe largitionum, the "hope of largesse." He does not mention the Italians at all.

The problem here is we have next to no information and what we've got is totally contradictory. It would be wrong, I think, to suggest that Drusus was particularly popular with anybody at all at the time of his death. Pretty much all our sources agree that he had pissed off both the equites and senatorial class, and those that recognize the distinction between the urban and rural plebs note that he had alienated the Italians with his colonial proposal. Our sources note that only the urban plebs still backed him, and realistically I don't think Appian can be right about that--in the later second century and early first century any motion proposing Italian enfranchisement was opposed tooth and nail by the urban plebs, whose already minor political position was essentially wiped out in the wake of the Social War, which rendered the tiny electoral power of the four urban tribes insignificant. Generally speaking scholars have tended to favor Appian's account. It's the one that makes the most sense, is the most detailed, and fits pretty well into a broader narrative of evolving Italian unrest. It seems likely that Drusus' bills were all proposed at once in the attempt to unify the voting blocs in a single go--introduced piecemeal they would seem to have little logic. There's also the problem of whether the laws actually passed. Appian (and Badian) seems to think they were not, but we have an indication from Cicero that the leges Liviae were abrogated by Phillipus. If they were passed we're not sure how--Livy and Florus seem to indicate that Drusus employed violence to force the passage of the bills, but there's not a hint of political violence in Appian and Velleius, and both Livy and Florus seem to be reciting stock invective against popularis tribunes. We simply don't know. More recently Mouritsen has suggested that the Italian bill was, as wikipedia and Mccullough suggest, proposed in a later package to try and salvage his earlier program. Perhaps, and Mouritsen's arguments make a lot of sense, but of course that raises other questions. I would be very uncomfortable saying, as you seem to think, that even if Drusus had anything resembling widespread urban popularity (which I sincerely doubt) this would be enough to ensure the passage of much of anything. With senatorial and equestrian resistance rallied against him the influence of the urban plebs would have been almost totally insignificant. Moreover, if Mouritsen is right and the Italian bill was proposed later, we should imagine Drusus as realizing the power of Italian influence. The Gracchi and Saturninus had both relied much more heavily on Italian support than urban, and in general the urban plebs were pretty unimportant--when tribunes like Gaius Gracchus proposed bills on their behalf they generally seem to be intended to keep them quiet and allow the much larger Italian base of support to get its way, a tactic which Drusus (tribune 122) had exploited to knock Gracchus' support out from under him. If Drusus had failed to take the Italians' influence into account earlier surely he realized his mistake now--moreover, championing Italian enfranchisement might actually secure him some important senatorial support, since the great orator Crassus was a vigorous supporter of Italian enfranchisement, and his death was a major blow to Drusus' program. But then again, all of this would be true if the bill were proposed earlier. Mouritsen's arguments for a later Italian bill are all good, but I think he overplays the damage a controversial Italian bill would do if it was indeed proposed alongside the rest of the package. Crassus had been lobbying for such a bill for a couple of decades now, and Badian has shown that the equites were sharply divided over the issue, with a very strong portion championing it. Italian enfranchisement was the topic up until the Social War, and I'm not totally convinced that its controversial nature alone would have been reason enough to avoid the subject. Still, we really don't know. Our sources are few and confused--and, importantly, information on the allegiances of various parts of the urban and rural plebs is hard to come by. We do not know with any degree of certainty what portions of the urban plebs and Italians supported or did not support the bill, these groups are spoken of in grand chunks, whereas we know the senate and equites were sharply divided on such controversial measures--should we not expect the plebs to be the same?