For three weeks Bernie Sanders has been on the uncomfortable receiving end of the Elizabeth Warren boomlet. Several polls have shown his national numbers in the Democratic primary field slipping from the high to the low teens; one poll showed Sanders falling behind Warren for the first time in an early-voting state, Nevada. So Sanders is fighting back with…a different poll. The subject line of the fundraising email reads “Bernie beats Trump. Bernie beats Trump”—eight times, just in case the message isn’t immediately clear. Never mind that the same Daily Beast/Ipsos survey shows Warren, Joe Biden, and Kamala Harris also knocking off the president.

Pointing to favorable data is a basic campaign tactic. Sanders’s next attempt to regain momentum will be more dramatic. On the second night of the first round of Democratic National Committee–organized debates, the Vermont senator will be standing center stage, right next to former vice president Biden. Sanders is likely to use a significant amount of his time to go directly after the front-runner, emphasizing the contrasts in their economic policies and political styles. “The debate is an opportunity for Senator Sanders to explain how his vision is different from everybody else’s, and one of the things you’ll see is unfettered capitalism having its reckoning,” Faiz Shakir, Sanders’s campaign manager, tells me. “How do people intend to deal with the social ills that result from corporate greed? And if your idea is, ‘Well, we just need to strike a middle-ground compromise and tweak at the margins and work with the private sector to try to do nice things for people’—we’ve tried that, and it has failed.” Will Sanders be specific in his criticisms, citing, say, Biden’s long-running backing of credit card companies? “There will be no flinch on his part,” Shakir says. “He’s not going to engage in mudslinging, but there are at least four areas where he’s raised policy disagreements with Joe Biden: the Iraq war, trade deals, climate change, health care. Tepid middle-ground status quo politics is failing the American public. We’ll see that play out in the debate.”

Sanders may well be able to poke holes in Biden’s support. One question is whether he’d be the most likely beneficiary of those defections. A recent Morning Consult poll showed him as the second choice of 29% of Biden’s voters, more than double the number for Warren and Harris. Some strategists are skeptical. “When this campaign started it was possible to see Bernie taking second or even beating Biden in Iowa and turning it into the same kind of race as Bernie vs. Clinton in 2016,” says Joe Trippi, who ran Howard Dean’s insurgent 2004 presidential campaign and was the media strategist for Doug Jones’s 2017 win in Alabama. “But that’s not how this is going to play out. It isn’t going to be him getting Biden one-on-one. There are too many other good candidates. He can contrast himself with Biden, but Biden’s voters aren’t going to move to Bernie. He has the money and the base to last the whole way, so he has to take a real long view of this and hope Warren falters.”

Focusing his fire on Biden might also help change the conversation from the growing, unhelpful Sanders vs. Warren narrative. Shakir is quick and clear in his complimenting of the Massachusetts senator, even though she has been siphoning poll support from Sanders. “Any success they’re having is a credit to the hard work that she and the campaign are putting into it,” he says. “They started off in a little bit of a difficult place and they put in some work and they started to move up. I just think these fluctuations in the polls are variants of smaller subsets of people, who are the ones paying attention and are saying that they’re going to vote and get through the polling screens. There’s nothing that makes us dispirited about the situation.”

Maintaining that diplomacy, though, is already proving difficult. A tweet from the official Sanders campaign account on Wednesday afternoon seemed to take an oblique shot at Warren as a new favorite of “the corporate wing of the Democratic Party,” unlike the real revolutionary that the Establishment fears, Bernie. (The Sanders campaign later said that its barb was aimed at centrist Democratic groups like Third Way.) Offline, however, Sanders allies characterize Warren as a technocrat, and see her current rise as the superficial product of a wave of glowing mainstream press attention. “When you start shooting up in the polls, people are going to come for you,” says Karine Jean-Pierre, a former Obama campaign and White House administration official who is now chief public affairs officer for MoveOn.org. “They are the two who have the closest ideology, and Bernie is struggling because there are more alternatives than there were in 2016. But he isn’t doing anything anybody else wouldn’t do.”

The intramural skirmishing probably won’t dent the proven, fierce loyalty of Sanders’s backers. And if the field stays fractured, reliable support in the high teens could be enough to win a formidable number of delegates. Shakir also points to the calendar, and to the fact that most Americans, shockingly enough, are not political junkies. “I believe strongly that the vast majority of people have not yet checked into this primary,” he says. “Maybe they’ll start by watching the debates. Maybe not. These are people who have a life and a lot of difficult things going on. I think it’ll start getting real in the winter, particularly if you’re a campaign that’s invested in mobilizing working people. Bernie doesn’t win if that mobilization does not occur.”