Loyal fans of Bernie Sanders have a difficult decision to make. If Hillary Clinton faces off against Donald Trump in the 2016 presidential election, legions of Sanders supporters will have to decide whether to switch allegiances or stand by Bernie until the bitter end.

At least some supporters of the Vermont senator insist they won’t vote for Clinton, no matter what. Many view the former secretary of state with her deep ties to the Democratic establishment as the polar opposite of Sanders and his rallying cry of political revolution. Throwing their weight behind her White House bid would feel like a betrayal of everything they believe.

These voters express unwavering dedication to Sanders on social media, deploying hashtags like NeverClinton and NeverHillary, and circulating petitions like www.wontvotehillary.com, which asks visitors to promise “under no circumstances will I vote for Hillary Clinton.” It’s garnered more than 56,500 signatures so far. Many feel alienated by the Democratic Party. They may want unity, but not if it means a stamp of approval for a political status quo they believe is fundamentally flawed and needs to be fixed.

“Just pack up your revolution and go home? Really? That’s not going to happen,” said Tara Margolin, a 50-year-old Sanders supporter and self-described Democrat who lives in Los Angeles. She dismissed the idea that Sanders voters might coalesce behind Clinton. “She would cement in place everything we are fighting against. I could never in good conscience vote for Hillary Clinton.”

The Democratic primary has been far less divisive than the Republican race. Trump, now the presumptive GOP nominee, has inspired some Republicans to tear up their voter registration, while others declare they would rather vote for Clinton. Insults and sparring reached a fevered pitch on the Republican side of the race that the Democratic candidates have not matched. More than two-thirds of Democratic voters even insist the race has done more to energize than divide the party, exit polls from recent primary contests indicate. Republican voters are far more likely to view the race as divisive.

The race has nevertheless laid bare a rift on the political left. Younger voters in particular have flocked to Sanders and his promises to take on Wall Street, break up the big banks, and root out corruption by stemming the tide of big money into politics. As she maintains her commanding lead in the race, effectively sealing off Sanders’s path to the Democratic nomination, Clinton has intensified her efforts to win over the senator’s supporters. “We will unify our party to win this election,” Clinton recently told a cheering crowd, adding: “There’s much more that unites us than divides us.”

Yet some of the voters who stand with Bernie have not been moved. “She’s lost my trust, frankly,” said Matt Brownfield, a 31-year-old Sanders supporter living in South Carolina. “I don’t think there’s anything Clinton could say or do that would change my mind.” Brownfield plans to write in the senator’s name when it comes time to cast a ballot in the general election if Sanders fails to secure the nomination. The decision wasn’t easy. “The thought of Trump becoming president is vile and disgusting,” Brownfield said. “But I realized I couldn’t live with myself if I voted for Clinton. I would say I’m Bernie or Bust.”

Some high-profile Democrats have voiced anxiety over party unity, but doom-and-gloom predictions over party divisions should be taken with a grain of salt. A McClatchy-Marist poll released in April found that roughly 25 percent of Sanders supporters say they won’t back Clinton in a general election if it came to that. But that estimate of discontent, and others like it that attempt to take the temperature of voters while the primary race is still underway, is likely overinflated, or at the very least a poor gauge of what will happen in the general election.

Overall, people are not very good at predicting what they will do in the future. “People have ideas about how they’re going to behave that don’t necessarily end up being the case when they’re faced with that eventual reality,” said Kim Nalder, a professor of government at California State University, Sacramento. “In the abstract we see things very differently from when we’re actually faced with a decision. We think we’re going to eat healthy and then we see a piece of a cake.”

Voters’ convictions can shift quickly as political circumstances change. Sanders supporters view Clinton as their opponent now, but once the general election arrives the battle lines will be re-drawn and that is likely to impact voter loyalty. “We don’t like to think of ourselves as contradictory and inconsistent, but we are,” said Lilliana Mason, a political science professor at the University of Maryland College Park. “When we’re in the middle of a fight, it’s really hard for us to imagine joining the other team. But who we identify with can change quickly when our attention shifts to a different fight.”

People tend to vote according to partisan affiliation in general elections. Of course, Sanders has done well with independent voters. But since many of those voters align with a particular party, the left-leaning ones are still likely to vote Democratic in the general election. “For many Sanders supporters there will be an early stage of grief, then there might be denial and anger,” Nalder said. “It takes a while before people get to acceptance.”

None of this is to say that voters who make a vow never to vote for Clinton won’t stick with it. They could write Bernie’s name in during the general election if she wins the nomination. They could vote for another candidate apart from Clinton, or decide to stay home. There will also be voters who emerge from the election with a far worse opinion of the Democratic Party. “When I voted Democrat for Barack Obama, I didn’t feel like the party was as corrupt as I feel like it is today,” said Bailey Osborne, a 54-year-old Bernie supporter from Burlington, Vermont, who says she is currently registered as a Democrat. “Now I feel absolutely no loyalty to the party.”

As the race drags on, some Sanders supporters feel frustrated by pressure from friends and acquaintances who want them to vote for Clinton if it comes to that. They argue that Sanders is a stronger candidate, pointing to polling indicating that he would be more likely to defeat Trump, despite the fact that early predictions of general-election matchups are unlikely to act as a reliable gauge of how candidates might fare. “People are acting like we want to throw the election, and that’s not the case at all,” Osborne’s 22-year-old daughter Casey said. “They say our unwavering support is a bad thing, but I think it’s a testament to the sort of loyalty he inspires. The reason that we are unwavering is that Bernie Sanders is a stronger candidate than Hillary. If anything it should be Clinton supporters that switch sides to Sanders.”

Sanders has made it very clear that he believes Trump must be defeated, promising to do “everything in my power to make sure that no Republican gets into the White House.” He has also vowed not to mount an independent presidential bid. But if he doesn’t win the nomination, Sanders has indicated Clinton will have to work hard to win over his supporters. “I can’t snap my fingers and tell people what to do,” Sanders told ABC News in a recent interview, adding: “If Secretary Clinton is the nominee, she is going to have to make the case to the American people, not just to my supporters, but all Americans.”

How can Clinton win over skeptical voters who have been loyal to Sanders? One way might be to pick a vice-presidential candidate with progressive credentials. If Clinton ends up as the nominee, but allows Sanders to play an influential role in shaping the Democratic Party platform, as she has indicated she may do, that could help to build goodwill. Still, the scars of the primary season won’t simply disappear for Democrats when the general election arrives. Clinton will inevitably confront voters whose negative opinions of her are fixed in place. For many Democrats, however, the general election will be a time to close ranks. In the end, the prospect of a Trump presidency may prove to be a powerful unifying force for the party.