The second Super Tuesday of 2016 was a disappointing evening for Bernie Sanders. After upsetting Hillary Clinton in Michigan a week ago, there was hope he would have the momentum to beat the odds again. Instead he lost in Florida, Ohio, and North Carolina, and he’s neck-and-neck with Hillary Clinton in Illinois and Missouri. If tonight ends up a clean sweep, or even 4-1, then the senator has to do a gut check: If he can’t win in Ohio and Illinois, then what is the purpose of his insurgent campaign?

If his only goal is to win the nomination, then it might be time for him to call it quits. But if his goal is to remake the Democratic Party by creating a powerful faction to the left of Clinton, then Sanders has every reason to stay until the end—and doing so could help Clinton defeat the likely Republican nominee, Donald Trump.

As Sanders said in his speech on Tuesday, his campaign has “defied all expectations.” This is undeniable. Clinton had the advantage of name recognition, endorsements from party officials, and ready access to wealthy donors. Still, a 74-year-old democratic socialist from Vermont has beaten Clinton in nine states, and the election calendar suggests he could win even a few more in the coming weeks. But on Tuesday he lost ground in the delegate count when he desperately needs to close the gap.

As long as Sanders is in the race, though, Clinton can’t take her left flank for granted and shift rightward in anticipation of the general election; doing so would risk embarrassing losses to Sanders. And the more delegates Sanders has, the bigger voice he will have in crafting the party’s platform at this summer’s Democratic convention. But there’s another reason for Sanders to stay in the campaign: Trump. Even if Sanders doesn’t win the nomination, he can still use his campaign platform to influence how the Democrats respond to Trump. 

Based on their words and actions so far, Clinton and Sanders have very different views on how best to defeat Trump in November. With her rhetoric of “make America whole,” Clinton seems set on defeating the divisive real estate mogul with a centrist message that might appeal to disaffected Republicans, particularly college-educated suburban voters who are likely to be put off by Trump’s extremism. Clinton’s effusive and controversial praise of Nancy Reagan seems part of this strategy. Doing so, however, would be to write off the white working class voters in Trump’s camp.

Sanders’s greatest impact on the Democratic primary has been to tug Clinton to the left on economic issues. He likely won Michigan by hammering home his opposition to free-trade agreements, and Clinton seems to have picked up the message: In Tuesday night’s speech, she said that no one “takes advantage of us—not China, not Wall Street,” words that echoed both Sanders and Trump’s rhetoric on trade and the outsized influence of corporate America.

Trump’s likely nomination gives Sanders a strong incentive to continue in the race— not only to pull Clinton to the left on economic issues, but to argue that her pursuit of well-to-do Republicans is a mistake. This strategy would essentially cede the white working class to Trump, which is risky not only in immediate electoral terms but fraught with danger for the country. If Democrats don’t make a pitch to win back the white working class, they will become ever more alienated and susceptible to the next Trump-style demagogue who comes around. Sanders-style economic populism offers a chance to peel away these voters from Trump, dooming any chance he has of defeating Clinton in November.

Here, then, is Sanders’s new mission: to be the spokesman for the Democratic Party’s alternative to Trumpism. That’s reason enough to stay in the race.