1. The decisive victory by Donald Trump in the Indiana primary knocked Ted Cruz out of the race and confirmed Trump as the all-but-certain Republican presidential nominee. The magnitude of Trump's victory compared to previous results in neighboring midwestern states suggests that he has gained popular momentum over the past month or so, which would have put him well within reach of a pledged delegate majority even if the margin in Indiana had been closer than it was. With Cruz departing the contest and John Kasich receiving an embarrassing 8 percent of the vote in a state adjacent to his own, Trump now faces an open path to a sweep of the remaining states on the primary calendar and an uncontested first-ballot victory at the national convention.
2. While he had previously declared that the Indiana results would be decisive, Cruz's exit in the immediate wake of his defeat was not universally expected. But Cruz, unlike Kasich, is young enough to consider seeking the presidency again in 2020 or thereafter. Staying in the race with little prospect of victory might only alienate Republicans whose support Cruz might wish to seek in a future contest. A guaranteed Trump nomination is also a better outcome for Cruz, in a strategic sense, than the possibility of an open convention throwing the nomination to an alternative compromise candidate. Trump will probably lose in the fall, and Cruz can run again in the future on the premise that the party suffered defeat by failing once again to nominate a true, principled conservative.
3. All the best evidence that can be brought to bear on the question indicates that Trump begins the general election with little probability of victory. Of course, dissenters will reply that few political experts foresaw Trump's nomination in the first place. But primaries are much more unpredictable than general elections, and Trump's political weaknesses are more vulnerable to attack by Democrats than by fellow Republicans. The complicated strategic dynamics of the multi-candidate Republican nomination race allowed Trump to escape being the target of a sustained negative campaign, but the Clinton campaign and allied Democratic groups will begin firing attacks in his direction immediately, hoping to "define" him quickly as an unacceptable candidate.
4. Yet a long campaign contains inevitable ups and downs in the standing of the candidates, as measured by public opinion surveys or as sensed by the political pundit class. Any signs of competitiveness or "tightening" will probably be heavily publicized by the segment of journalists who find stable races boring, are unimpressed with Hillary Clinton, and/or view Trump as having the potential to fundamentally reorder the electoral coalitions of the two parties. It is also clear that Trump is, in effect, judged by a different set of standards than other candidates; if Clinton or Obama or Mitt Romney had personally accused a rival candidate's father of associating with Lee Harvey Oswald based solely on a report by the National Enquirer, for example, it would be the biggest media story of the month and widely treated as a self-evidently disqualifying catastrophe. Expect much of the press to leap on any sign that Trump has become more "serious" or "presidential" over the course of the campaign—not because journalists are intentionally slanting coverage to favor Trump, but because change is always a better story than more of the same and "both sides do it" is often the default presumption.
5. We should not make too much of declarations from Republicans at this stage of the race that they will not vote for Trump in November. No doubt some disaffected partisans will indeed refuse to support him, though staying home or skipping over the presidential race on the ballot are both more likely forms of Republican protest than actually crossing party lines to vote for Clinton. But general election campaigns are usually effective at rallying partisans around their nominee—if only by reminding them of what they dislike about the opposition—and, unless Trump completely implodes, he is likely to gain the support of the vast majority of Republican identifiers who participate (with the possible exception of Republican Latinos, who may defect at higher rates). Even if Trump suffers a decisive defeat, this residual party loyalty will prevent the Democratic opposition from winning a double-digit victory in the popular vote or carrying more than 30–32 states. There is almost no chance of a true national landslide on the scale of 1964, 1972, or 1984 in today's highly partisan electoral environment—especially when the Democratic nominee is not particularly popular in her own right.