Bernie Sanders, at a campaign rally in Louisville. PHOTOGRAPH BY JOHN SOMMERS II / GETTY

With Donald Trump more or less wrapping up the Republican nomination on Tuesday night, the fact that Bernie Sanders also scored a big victory in Indiana was somewhat overlooked. The Vermont senator overcame a sizable deficit in the opinion polls to finish ahead of Hillary Clinton by almost seven percentage points in a heartland state that she carried in 2008. It was his biggest upset since he won in Michigan, in March.

In a series of interviews on Wednesday, Sanders confirmed that he will stay in the Democratic race until at least June 14th, when the final primary will be held, in Washington, D.C. Sanders also said that he would try to win over Democratic superdelegates who are currently committed to his opponent, a strategy that could extend the contest until the Party Convention, in Philadelphia, at the end of July. “I think we have got to make the case that the superdelegates, who in many cases were onboard [with] Hillary Clinton even before I got in the race, that they should take a hard look at which candidate is stronger against Donald Trump,” Sanders said to NPR’s Steve Inskeep. “And I think we can make that case.”

Sanders’s determination to press on is causing consternation in the Clinton camp, and in the Democratic Party establishment. Eager to avoid alienating Sanders’s large body of supporters, most Party figures have avoided publicly calling on him to quit, but some are citing Trump’s victory as a reason to unify behind Clinton. “If [Sanders] wants to stay in and discuss the platform, that’s obviously his right,” Elaine Kamarck, a senior fellow at the Brookings Institution who served in Bill Clinton’s Administration, told the Washington Post’s Greg Sargent. “But at this point, we’re on the verge of a choice between someone eminently sensible and qualified, and someone who is a real wild card. It’s a little frightening having him out there trying to take down the sensible candidate.”

In the weeks ahead, the calls for Sanders to wrap up his campaign are likely to become more explicit. He seems certain to ignore them, and he has at least four reasons to do so. First, most of his supporters want him to keep going. Second, he still has a (very) slim chance of obtaining the nomination. Third, there isn’t much evidence that his dropping out would affect the result in November. And fourth, back in 2008, Clinton herself did something very similar to what Sanders is doing now, extending her primary contest with Barack Obama well beyond the point at which most commentators had concluded that she had no chance of winning.

The result in Indiana confirmed what we already know: Sanders is very popular among younger Democrats and Democratic-leaning independents, and particularly among white voters. According to the network exit poll, he carried the under-forty-five demographic by sixty-eight per cent to thirty-two per cent, and he won independents by seventy-two per cent to twenty-eight per cent. (The Indiana primary was an open one.) As usual, Clinton performed strongly among older and non-white voters, and among self-identified Democrats.

Since the primary season began, Sanders has won more than nine million votes and finished ahead of Clinton in eighteen states. (Clinton has won more than twelve million votes and won twenty-three states.) Sanders continues to attract large crowds—on Thursday he will be campaigning in West Virginia—and he seems likely to win more primaries in the coming weeks, including in West Virginia, on May 10th, and Oregon, on May 17th. If he were to end his campaign now, many of his supporters would be furious, and even some Democrats who aren't necessarily backing him would be disappointed. According to new poll from NBC News/Survey Monkey, fifty-seven per cent of Democrats and Democratic-leaners want Sanders to campaign until the Convention, and just sixteen per cent think he should drop out now. Eighty-nine per cent of Sanders's supporters said they wanted him to keep going until July. More surprisingly, perhaps, twenty-eight per cent of Clinton's supporters agreed.

According to Predictwise, a Web site that combines data from the betting markets and opinion polls, the probability that Sanders will win the nomination is just two per cent. That figure reflects the fact that, including superdelegates, Clinton is less than two hundred delegates short of getting the twenty-three hundred and eighty-two she needs to clinch the nomination. With more than nine hundred delegates available in the remaining thirteen contests, most of which will award delegates proportionally, she is well-nigh certain of reaching her target.

That’s the reason many commentators have declared the race over. Sanders, however, is pinning his hopes on persuading large numbers of superdelegates to switch sides. To have any chance at all of winning them over, he would need to overcome Clinton’s lead in elected delegates, which currently stands at seventeen hundred and one to fourteen hundred and eleven, according to a tally by the Times. That would require him to win about two-thirds of the remaining delegates, a mighty task, but one that isn’t completely beyond the bounds of possibility. Unless and until it becomes so, Sanders has reason to keep going. “We think we have a path toward victory," he told Steve Inskeep. "Admittedly, it is a narrow path.”

When interviewers pressed Sanders on whether he was doing the Republicans a favor by staying in, he insisted that, by continuing to engage young voters and independents, he was helping his own Party’s chances. “The way Democrats win elections is when the voter turnout is high, when people are excited. And that’s what we are doing,” he said to Scott Pelley on Wednesday’s edition of the “CBS Evening News.”

Clinton’s supporters acknowledge that she needs to attract more young voters, which were a key part of the Obama coalition that carried the Democrats to victory in 2008 and 2012. Their complaint is that Sanders, in attacking Clinton on issues like trade and her connections to Wall Street, is providing ammunition for Trump—an argument that contains some truth. In an interview with Lester Holt, of "NBC Nightly News," on Wednesday, Trump mentioned Sanders’s claim that Clinton wasn’t qualified to be President, twisting it a little to serve his own ends.

But even if Sanders dropped out today, Trump would continue to repeat some of the criticisms that Sanders has levelled at Clinton. That is how Presidential campaigns always work. In 2008, the McCain campaign tried to cast Barack Obama as a young and untested politician who wasn’t ready to be President, a line of attack that Clinton had pursued in the Democratic primary. It didn’t work for McCain, and it won’t necessarily work for Trump.

Some Clinton supporters are pointing to the damage that Ted Kennedy did to Jimmy Carter during the 1980 primary, but that was a different sort of race. Kennedy challenged a sitting President. This year is much more like 2008, when it was Clinton who refused to drop out despite having no apparent path to victory, to the consternation of some Democrats. During April and May of that year, she continued to campaign aggressively, and she also tried to win over superdelegates. When reporters brought up the argument that she should bow out to insure Party unity, she reminded them that, in 1992, her husband, Bill, didn’t wrap up the nomination until the middle of June.

On Wednesday, Sanders repeated his pledge to keep going “until the last vote is cast,” but it isn’t quite clear whether he was referring to primary votes, those of superdelegates, or both. If, in mid-June, when the primaries are done, Clinton still has a significant lead in elected delegates, I would expect Sanders to end his candidacy and endorse her. If he has somehow overcome the odds to draw even in elected delegates or take a slight lead, he will surely carry on to the Convention. Given how far the Sanders campaign has come, such a strategy doesn’t seem unreasonable.