CRESCO, Iowa — Former Vice President Joseph R. Biden Jr. retains a lead among likely Iowa caucusgoers, but both he and Senator Bernie Sanders have lost ground over the past three months while Senator Elizabeth Warren and Pete Buttigieg, the mayor of South Bend, Ind., have made clear gains, according to a new poll from The Des Moines Register and CNN.
The poll showed that Mr. Biden is the first choice for 24 percent of would-be Democratic caucusgoers, compared with 16 percent for Mr. Sanders of Vermont, 15 percent for Ms. Warren of Massachusetts and 15 percent for Mr. Buttigieg.
In March, before Mr. Biden formally entered the race, he and Mr. Sanders held a commanding lead, with 27 percent and 25 percent support according to The Register’s polling, which has long been judged by campaigns as the best in the state. At the time, Ms. Warren was polling at 9 percent and Mr. Buttigieg at 1 percent.
Senator Kamala Harris of California was at 7 percent support in both surveys.
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The latest poll results reflect a race that is as wide open and unsettled as any Democratic presidential contest in decades.
The poll comes on the eve of Iowa’s first major political event of the summer campaign season — a state Democratic Party gathering Sunday in Cedar Rapids that 19 of the 23 presidential candidates are expected to attend.
Mr. Biden is skipping the event, electing to remain in Washington before a two-day Iowa swing beginning Tuesday, when President Trump will also be in the state. Mr. Biden’s campaign is emerging from a rocky stretch in which he reversed a long-held position against federal funding for abortions after facing intense pressure from fellow Democrats and within his own campaign.
For months, Democratic officials in Iowa have said Mr. Biden’s support in polls overrepresents the level of organization or excitement for his candidacy among the party’s most committed activists.