Senator Elizabeth Warren’s slip in the 2020 primary polls has been accompanied by a dip in donations, with her campaign setting a rare public goal: aiming to raise $20 million for the fourth quarter of 2019 ending Tuesday, or about 20 percent less than what she raised in the previous three-month period.

Former Vice President Joseph R. Biden Jr., meanwhile, has rebounded from a weak third quarter, in which he raised only $15.7 million and spent $2 million more than he took in. Now his campaign is trying to assert his front-runner status in the Democratic primary, pushing in the final 48 hours of the year to post “our biggest fund-raising quarter yet,” as Mr. Biden wrote in an email on Sunday, by topping the $21.5 million he raised last spring.

The shifting financial fortunes of Ms. Warren and Mr. Biden illustrate the unsettled nature of the Democratic presidential contest heading into 2020, with four candidates — Mr. Biden, Ms. Warren of Massachusetts, Senator Bernie Sanders of Vermont and Mayor Pete Buttigieg of South Bend, Ind. — battling for position in the top tier of polling and seeking to bolster their treasuries ahead of the final sprint to the Iowa caucuses and beyond.