Sanders' haul -- which brings his 2015 total to $73 million -- was almost exclusively buoyed by his prodigious online fundraising operation. Aides told CNN that Sanders received over 2.5 million donations in 2015 from 1 million different individuals.
Clinton's campaign announced Friday that it raised $37 million for the primary campaign
in the last three months of 2015, easily surpassing the $100 million goal the campaign had set earlier in the year.
"This people-powered campaign is revolutionizing American politics," Jeff Weaver, Sanders' campaign manager, said in a statement on Saturday. "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. We are making history and we are proud of it."
The final quarter of 2015 also saw Sanders dramatically increase his spending. According to aides, the campaign ended 2015 with $28.4 million cash on hand. That is only $2 million more than the $26.2 million the campaign had in the bank at the end of the third quarter of 2015, meaning Sanders' operation spent the bulk of what they raised in the fourth quarter.
Much of that spending, according to aides, was focused on building infrastructure in early primary states, including deploying organizers to South Carolina and Nevada, and building the campaign's already existing organization in Iowa and New Hampshire.
For the year, Sanders' campaign spent 61% of the money it brought in.
Sanders has long been a formidable fundraiser with deep roots in online and small donor fundraising, but even some of his closest aides were unsure he could raise the money needed to run a presidential campaign when the launched earlier this year. Tad Devine, his top strategist, said that the operation had hoped to bring in between $40 and $50 million by the end of 2015.
Sanders smashed that goal, but in a dramatically different way than Clinton did.
A bulk of Clinton's haul came from fundraisers she or her husband, former President Bill Clinton, personally headlined. The Democratic front-runner headlined a total of 58 fundraisers in the fourth quarter, a pace identical to the 58 events she headlined in the second and third quarters of 2015. And the former president headlined dozens of fundraisers in 14 states and Washington, D.C., including events in Texas, Wisconsin and Ohio, during the fourth quarter.
According to CNN's analysis, fundraisers personally headlined by Clinton brought in at least $22 million in the forth quarter.
Sanders, however, only headlined two fundraisers in the last three months of 2015, bringing his year total to nine, while Clinton's total for the year was 174.
What's more, almost all of Sanders' donors have yet to reach the $2,700 max donation, according to aides, who said Saturday that only around 600 Sanders supporters have maxed out.
That means Sanders' campaign can continue to hit up their donors for small donations as the primary calendar moves along.
Sanders' campaign has also raised big money at notable moments in the fourth quarter. Sanders' aides bragged that the campaign raised $2.5 million in the two days after the first Democratic debate in Las Vegas.