Hillary Clinton's campaign manager Robby Mook acknowledged Thursday that rival Bernie Sanders drew in a larger fundraising haul than the former secretary of state in the month of January.

"He outraised us in January," Mook said, at a Bloomberg Politics breakfast Thursday. "We've been very candid with our online supporters that Senator Sanders' online supporters have given more often -- a higher proportion of his list contributes.

Later Thursday, Clinton's campaign announced that Clinton had raised $15 million for her primary campaign in January, compared to the over $20 million raised by Sanders' campaign. Sanders collected donations from over 770,000 individual contributions, with online donations averaging $27 apiece. Clinton's campaign said 95 percent of its donations in January were in amounts of $100 or less.

On Sunday, Sanders campaign manager Jeff Weaver criticized Clinton in a statement touting his candidate's fundraising numbers.

• Is Hillary Clinton a progressive? Can Bernie Sanders win in November?
• Hillary Clinton: The vast, right-wing conspiracy" is "even better funded" now
• Bernie Sanders calls faith a "guiding principle in my life"

"As Secretary Clinton holds high-dollar fundraisers with the nation's financial elite, our supporters have stepped up in a way that allows Bernie to spend the critical days before the caucuses talking to Iowans about his plans to fix a rigged economy and end a corrupt system of campaign finance," Weaver said.

In past fundraising quarters, Sanders has never outpaced Clinton's totals. But in 2015, from July to September, Sanders came close: He raised $25.5 million in that third quarter, just $3 million behind his rival's campaign.

The two Democratic candidates are set to meet Thursday night in Durham, New Hampshire for their first one-on-one debate since the Iowa caucuses, where Clinton squeezed out a narrow victory.

Sanders and Clinton will face off in next week's New Hampshire primary, scheduled for Tuesday, Feb. 9.