For months, Biden advisers had said they just needed enough cash to compete, and all the way through the end of February, Mr. Biden was racing to conserve that cash.

He spent only $13.1 million last month — less than the billionaire Michael R. Bloomberg was spending on average per day. During the same period, Mr. Sanders spent more than twice as much on digital and television ads alone as Mr. Biden spent on his entire campaign operation — his payroll, his ads, his consultants, his events.

In fact, Mr. Biden spent the least money in February of the seven candidates who had qualified to be on the debate stage. For the duration of the campaign, Mr. Biden ranks as only the sixth-biggest spender in the Democratic field as of the end of February. The weak fund-raising in Mr. Biden’s first 10 months as a candidate — he had only $12.1 million in the bank entering March — could be a concern for Democrats headed into a general election against President Trump’s campaign operation, which has steadily banked money for the last three years.

Mr. Trump’s campaign alone — not counting his joint operations with the Republican National Committee — reported more than $94 million still in his treasury at the end of February, more than Mr. Biden had raised in total since he entered the race in April 2019.

In addition, the coronavirus pandemic has put on hold indefinitely Mr. Biden’s ability to host in-person fund-raisers and is widely expected to sharply slow the pace of small online donations, as well.