Sanders and Warren, campaigning on promises to enact some form of Medicare for All, free public college, and a wealth tax, have delighted the leftmost segment of the Democratic base. Warren, with her steady stream of ambitious policy plans, has drawn consistently massive crowds that happily chant wonky slogans. Sanders raised $34.5 million in the last three months of 2019—far more money than any other presidential candidate. And more than 26,000 people attended Sanders’s October rally with Representative Alexandria Ocasio-Cortez of New York, making it the largest event held by any Democratic presidential candidate this cycle.

Biden’s rallies are consistently much less well attended. And although crowd size is not necessarily predictive of electoral success, it could indicate whether a candidate has a sizable pool of enthusiastic volunteers to draw from in the general election. A Biden nomination would trigger a huge deflation in enthusiasm, and a shrinking of that volunteer pool, progressives argue. “If a candidate that gets selected doesn’t have the type of energy and excitement from the troops—the people who give small dollars, the people who phone bank, who show up to rallies—it will be harder” said Rashad Robinson, the president of the racial-justice organization Color of Change.

Read: The kumbaya candidate

For many Democrats, that warning triggers an unpleasant flashback to 2016. Sanders, after losing the primary, was late to endorse Hillary Clinton. At least 20 percent of the people who voted for Sanders in the primary did not vote for Clinton in the general election against Trump, according to one study. But every progressive organizer and leader I talked with for this story told me a variation of the same thing: They’re not concerned that Americans will choose Trump over Biden. They’re worried that, absent a Democratic candidate who excites them, many Americans might not vote at all.

Democrats have two theories of how to win the 2020 presidential election: persuasion versus turnout. Advocates of the former, generally moderates, believe that Clinton lost to Trump mostly because she failed to convince enough moderate voters in swing states. But progressives say that an emphasis on turning nonvoters into voters is more important for a Democratic victory in November. They blame Clinton’s loss on failing to inspire and mobilize Americans: An estimated 4.4 million people who voted for Barack Obama did not vote in 2016.

This kind of mobilization strategy relies heavily on local canvassing, and some of the activists involved with grassroots progressive groups told me that they have serious concerns about being able to mobilize volunteers for Biden. Jackie Dempsey, a 53-year-old member of the Forest Hills, Pennsylvania, chapter of Indivisible, a progressive group, intends to campaign for Biden just as vigorously as she would for any other nominee. But when Dempsey asked other members of the group what they’d do if Biden was the Democratic nominee, she received a range of responses: “Some people said, ‘I’ll vote for him but I won’t work for him,’” Dempsey told me. “Some people said, ‘I’ll work around him.’ [Others said,] ‘I’ll make sure Democrats are registered, but I won’t even vote for him.’”