If he is able to harness anti-elite sentiment in the party’s small donor base to ride out the post-debate turmoil, it would put him in the company of more populist politicians such as Senator Bernie Sanders, Independent of Vermont, and Representative Marjorie Taylor Greene, Republican of Georgia. Those lawmakers have generated waves of cash from small donors by invoking perceived mistreatment at the hands of the establishment.

“We do know that high-visibility, high-intensity moments can trigger an immediate flood of small donations and that anger, resentment and outrage are powerful motivators in politics, including for small donors,” Richard H. Pildes, a law professor at New York University who has studied the role of small donors in fueling political polarization, said in an email. “It’s also possible we are seeing a conflict between the base of the Democratic Party, who want Biden to stay in, and the more ‘elite’ faction of large donors, who want an alternative.”

Small donors have long been valued in politics as an indicator of grass-roots enthusiasm, as well as a sustainable source of cash, since they can give repeatedly without reaching contribution limits. Advances in online, email and mobile fund-raising applications have allowed campaigns to use big events as opportunities to solicit contributions from supporters of average means.

Small donors tend to give “from the heart,” said Eitan D. Hersh, a professor of political science at Tufts University who has studied the motivations of political donors. “The bigger donors are donating more from the head than the heart,” he said, predicting that Mr. Biden’s campaign would experience “a retreat of big donors and therefore a relative gain in proportion of small donors.”

Carol L. Hamilton, a lawyer in Los Angeles who is a member of Mr. Biden’s national finance committee, said he had many supporters who “may not be million-dollar donors or hundred-thousand-dollar donors, but they care, and they’re going to vote, and right now they’re showing that they support our president by giving a few bucks — $5, $25, whatever it is they’re giving — to make a statement that they think that this president should stay in the race.”