Lucy Nicholson / Reuters

Bernie Sanders may be on the verge of a winning streak. After defeating Hillary Clinton in the Indiana Democratic primary, the Vermont senator notched a win in West Virginia on Tuesday, and appears well-positioned for a victory in the upcoming Oregon primary.

That will allow Sanders to claim momentum. It’s also likely to help the campaign pull in a fresh flood of small-dollar donations from energized supporters. Still, none of that changes the fact that Hillary Clinton has effectively sealed off a pathway to the nomination for Sanders. To get technical, it is no longer mathematically possible for Sanders to win enough pledged delegates in the remaining primary contests to win the nomination. On top of that, Clinton holds a commanding lead in superdelegates, a pool of influential Democrats made up of elected officials and party leaders. So why hasn’t Clinton managed to dispatch her Democratic rival?

The senator’s populist message of tackling income inequality and taking on Wall Street has inspired an ardent following. Voters are animated by a wide array of concerns, and the choice to stand with a particular candidate is a personal decision. But to understand why support for Sanders hasn’t dried up despite the long odds he faces, it may be helpful to describe a spectrum of voters. There are the realists, the casual fans, and the defiant die-hards.

The realists think Sanders probably can’t win, but plan to vote for him anyway. A CBS poll from April shows that 44 percent of Democratic primary voters want Sanders to win, but only 23 percent actually believe he will win. That suggests at least some people support the senator to indicate political preference, not because they necessarily believe their vote makes it more likely for him to win.

They may recognize that even if Sanders fails to secure the nomination, the more states he wins and votes he amasses, the easier it will be for him to exert influence on the Democratic party platform. They may hope to send a message with their vote, signaling that they don’t support the political status quo. Or they may feel that Clinton is a more viable general election candidate, but simply like Sanders better.

If that’s the case, Clinton’s success may free up even more Democratic primary voters to vote for Sanders without running the risk of jeopardizing Clinton’s shot at the nomination.  “For many people, voting is expressive as much as instrumental,” said Kim Nalder, a professor of government at California State University, Sacramento. “They aren't deluded. They are just more interested in taking the symbolic stand before facing the reality of Clinton vs. Trump.”

Then there are the casual fans. This segment of voters may be just as devoted to Sanders and his cause, but isn’t immersed in the ins-and-outs of presidential politics or well-steeped in the delegate math. For them, hearing Sanders say he stands a shot at winning upcoming primary contests, and seeing the senator continue to win states, provides enough motivation to get out and vote. They may be first-time voters or people who don’t frequently vote, but have nevertheless been inspired by Sanders to flock to the polls in 2016.

These voters aren’t likely to be cynical about the process. They trust the narrative spun by Sanders and supporters who take to social media to talk up his momentum in the race, and may not have even encountered predictions that he won’t win. “The presidential primary process in the U.S. is hopelessly byzantine,” Nalder said. “Only a handful of party insiders probably truly understand the details. What many Sanders supporters see is a candidate still in the race who still claims to have a path to victory.”

At the far end of the spectrum, there are the defiant die-hards. These voters willfully reject predictions from the mainstream media and political pundits that Sanders’s path to the nomination is effectively closed off. These are supporters who devote hours to pro-Sanders message boards, and furiously take to Twitter to denounce anyone who portrays the race differently than they do. Hearing people say Sanders can’t win may make these voters even more motivated to show support for him or to volunteer for his campaign in an effort to get out the vote. “Many of his supporters don’t trust the government establishment, but they also don’t trust the media or the polls,” Lilliana Mason, a political science professor at the University of Maryland College Park, said. “So for them, the reports of his demise are premature.”

These voters are armed with statistics to argue that Sanders is well-positioned in the race, but they may be engaged in wishful thinking. Ardent supporters often point to polling indicating that Sanders stands a better chance at defeating Trump than Clinton, despite the fact that many of these hypothetical match-ups aren’t likely to be reliable indicators of what will happen in November. Josh Putnam, a political science lecturer at the University of Georgia, calls the general election argument “misleading.” For one thing, Sanders has not been subject to the same kinds of negative attacks as Clinton, precisely because he has not been viewed as a frontrunner. If that were to happen, his standing in polls would almost certainly drop. “It’s a phantom advantage,” Putnam said.

The unexpected success of Sanders’s campaign to his point also gives these supporters reason to believe that he can continue to outperform expectations. “His electoral success so far has been something that no one predicted, which creates a sense of possibility that might be hard to shake, even in the presence of evidence that he is mathematically finished,” Mason added.

While Sanders may continue to win primary contests, many Democrats have already coalesced around Clinton. She not only has a commanding lead in the all-important race for delegates, she has also racked up a popular vote lead of more than three million votes. There are also indicators that most Sanders supporters would back Clinton if she faces off against Donald Trump in the general election. A CNN poll released in May found that 86 percent of Sanders supporters would vote for Clinton over Trump. People also tend to vote according to partisan affiliation. So while some Sanders supporters insist they will never vote for Clinton, there is reason to believe that many would be willing to switch allegiances if their favored candidate fails to secure the nomination.

For now, Clinton isn’t calling on Sanders to drop out of the race or stop campaigning. Still, a winning streak for the senator puts pressure on the Clinton campaign to expend resources on the primary race that might otherwise be directed against Trump. After it looked like the campaign might wind down its ad spending, Politico reported on Monday that the campaign has bought up airtime in Kentucky ahead of its upcoming Democratic primary. A prolonged primary race, combined with the fact that the Republican field has now cleared out leaving Trump as the last man standing, puts Clinton in the awkward position of having to fight a two-front battle. She must fend off attacks from Trump and Sanders, and attempt to win over centrists and progressives at the same time.

There are other risks for Clinton. Sanders has continued to suggest that that the Democratic establishment is playing favorites. That line of argument threatens to poison the well for Clinton if and when she does secure the nomination. Clinton also has to contend with high unfavorable ratings among voters, though not as high as Trump. In recent weeks, she has started to make appeals to Sanders supporters, noting that there’s “more that unites us than divides us.” But it remains to be seen how successful she will be in her effort to unify Democrats. For now, Clinton may be waiting for Sanders to exit the race before she attempts to aggressively court younger progressive voters. After all, as long as Sanders sticks it out, plenty of voters will remain loyal.

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