The pressure over money is bound to intensify as the leading Democratic campaigns compete in the early voting states and the extraordinarily expensive media markets of Super Tuesday states like California. Ms. Warren, currently Mr. Biden’s top rival, has already reserved $2.3 million more in television ads in the early voting states than he has, according to Advertising Analytics, a media-tracking firm . In the last 30 days, she spent twice as much as Mr. Biden on Facebook ads, while Mr. Buttigieg spent more than three times as much as Mr. Biden , who has begun buying more online ads after he sharply scaled back his digital spending over the summer.

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Mr. Biden and his advisers wave off any cash concerns, arguing he is already universally known, seen as Mr. Trump’s strongest challenger and beloved in a way money cannot buy. Mr. Biden, in an interview set to air Sunday on “60 Minutes,” gave a blunt answer about how he would overcome his rivals’ cash edge: “I just flat-out beat them.”

More than most candidate s, Mr. Biden has cultivated major Democratic bundlers who gather checks from friends and fellow supporters. To that end, Mr. Biden has used the old-fashioned strategy of giving fancy titles to his top bundlers: an “advocate” must raise $25,000, a “protector” $50,000 and a “unifier” $100,000. Perks, which vary by level, include monthly calls with campaign leadership, and invitations to finance committee retreats and forums.

Yet Mr. Biden has struggled to line up the vaunted Obama money operation behind him. Of the nearly 800 bundlers who raised at least $50,000 for Mr. Obama’s 2012 campaign — different because it was a presidential re-election, not a crowded primary — less than a quarter have donated to Mr. Biden so far this year, according to an analysis of Federal Election Commission data. An even smaller share are actively tapping their networks for him.

One bright spot is that Mr. Biden’s online donations have picked up of late. His best week for raising money since the second week in the race came in late September as the impeachment scandal exploded. Then, in the first half of October, he raised more online than he had raised in all of September, the campaign said .

The campaign has employed some sky-is-falling tactics to spur giving, including an email last week threatening “budget cuts” if more money did not materialize. “Low on resources” was the subject line of a Friday missive. (No cutbacks are planned, the campaign said.)

If Mr. Biden’s fund-raising does not improve, he could be hard-pressed to ramp up his operations for the primary season. Just to pay for his existing campaign infrastructure through the Iowa caucuses would require the $9 million he has in the bank now plus the full fund-raising haul that he had in the third quarter.