Sanders's fundraising, which continues to be fueled largely by small online donations, ensures that he will be in a position to compete, however, with the party's front-runner through the first several nominating contests, which begin next month in Iowa and New Hampshire.
Aides to Sanders touted a figure showing that 99.9 percent of Sanders's financial supporters have given less than the legal maximum of $2,700, meaning they can be tapped again if Sanders performs strongly in the early caucuses and primaries. To date, they said, more than 1 million people have given to the campaign of the self-described democratic socialist, who was initially written off as a fringe candidate when he entered the race.
"What we are showing is that we can run a strong, national campaign without a super PAC and without depending on millionaires and billionaires for their support," said Sanders campaign manager Jeff Weaver. "We are making history, and we are proud of it."
Sanders entered the new year with $28.4 million in the bank for his primary campaign, compared to the nearly $38 million the Clinton campaign said it had on hand as of Dec. 31.
While Sanders raised money at a greater clip than he had previously, he also managed to spend nearly as much as he raised: an estimated $31.7 million — or about 96 percent of the amount he took in during the final quarter of the year. Aides said much of that went to ramping up operations in early nominating states around the country.
After getting a slower start than Clinton on raising money, Sanders has now come close to matching her haul for the primary campaign during the past two fundraising quarters.
For the full 2015 calendar year, Clinton reported raising a total of $112 million, while Sanders said he has raised $73 million.
While Clinton has coordinated with the National Democratic Committee to raise an additional $18 million, Sanders has not replicated those efforts, despite an arrangement with the national party that allows him to do so.
Sanders and the DNC have had a turbulent relationship in recent months, punctuated by a lawsuit Sanders filed against the DNC in the wake of a controversy over a breach in a DNC voter database committed by several Sanders staffers.
"We remain happy to work with them," Sanders spokesman Michael Briggs said Saturday, when asked about joint fundraising efforts. "The party hasn't given us any dates for events."
Aides said Sanders has built a stable of 1 million donors more quickly than any other White House candidate. In 2008, Barack Obama did not report receiving a contribution from his one millionth donor until Feb. 27 of the election year, they said.
Sanders's standing in the race is likely to hinge on his performance in Iowa, which holds its first-in-the-nation caucuses on Feb. 1. Recent polls have shown Sanders trailing Clinton there but with a lead in New Hampshire, a state that neighbors Vermont.
Matea Gold contributed to this story.