How candidates are raising their money — and whom they solicit for contributions — has become a central point of contention in the primary in recent weeks, with Mr. Sanders and Ms. Warren touting their independence from the traditional world of big contributors while hitting Mr. Biden and Mr. Buttigieg for their reliance on wealthy donors.

“For far too long, the wealthy and the powerful have used their money to buy our candidates and our elections,” said Tim Tagaris, a senior adviser to Mr. Sanders. “And what Bernie Sanders is proving — for the first time — is that one can run for president without begging them for their money.”

In a sign of the breadth of donors that Mr. Sanders counts, his campaign was pushing for 135,000 donations in the last two days of the year; Mr. Biden has set a goal of 500,000 donations for the entire quarter.

Ms. Warren, who like Mr. Sanders has decided not to make appeals to big donors at traditional fund-raisers, has called for her rivals to disclose their lists of “bundlers,” who have gathered campaign checks for them. In turn, Ms. Warren has been pressed about the $10 million she transferred to her presidential campaign from her 2018 Senate bid, when she was still courting such large contributors.

Both Mr. Biden and Mr. Buttigieg recently did disclose their bundlers, with Mr. Buttigieg sharing the names of more than 100 people or couples who had gathered at least $25,000 and Mr. Biden disclosing more than 200. Mr. Biden appeared to time his release to minimize attention, revealing the names of his top fund-raisers late on the Friday evening after Christmas.

Mr. Biden has seen some of the bundlers for his former rivals who have left the race — especially Ms. Harris but also former Representative Beto O’Rourke of Texas — gravitate toward him as he continues to lead in national polling. Former Harris supporters now backing Mr. Biden include John Emerson, an investment executive in California; two influential New York finance-world fund-raisers, Marc Lasry and Blair Effron; and Alex Heckler, an attorney in Florida.