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On the eve of the Iowa caucuses, Bernie Sanders’s campaign said Sunday is has already raised $20 million this month, an enormous haul that backs up the campaign’s assertion that it will have enough money for a long, hard-fought nominating fight against Hillary Clinton.

In an earlier interview, Tad Devine, a senior adviser to the campaign, said Mr. Sanders’s success raising vast amounts of money from small donors separates him from other liberal candidates who generated such enthusiasm in the past but who saw their early success peter out as money ran dry. Any early momentum in the early contests Mr. Devine said in an interview, would trigger even more donations.

“I’m talking tens of millions of dollars in a period of a couple of weeks,” Mr. Devine said.

To put that January fund-raising figure for the Sanders campaign in perspective, Hillary Clinton’s campaign said it raised $37 million for the last three months of 2015.