“Bernie Sanders out of any candidate running for president has been fearless in directly and explicitly confronting corporate power and the billionaire class,” David Sirota, speechwriter for Sanders, told The Daily Beast. “I think, ultimately, electability without concrete policy positions and a policy agenda then becomes this squishy thing that can move all over the place.”

That’s because Sanders’ team has bought into the polling data showing that the senator is listed more than any other candidate as the second favorite option for Biden supporters. To get those voters, the campaign is not just seeking to attack Biden’s voting record, but to draw a contrast in how the two have structured and funded their operations. The former vice president, who has embraced large-dollar fundraisers, has consistently stated that reaching across the aisle is one of his central strengths—though the recent blow up over his nostalgic reminiscence of working with segregationist senators may complicate that. Sanders, who has relied on small-dollar and more recently some grassroots fundraisers, has argued that a mass movement of voters will allow him to put pressure on politicians in his prospective administration.

“He’s hopeful that one of the ways to draw the contrasts with other candidates in the field is to talk about how you believe you intend to govern if you can’t build a grassroots movement,” said Faiz Shakir, Sanders’ campaign manager. “Put more directly, are you building a grassroots movement or not? And what role do you see for enlisting people in the course of policy change?”