A new national poll suggests that the race for the 2020 Democratic presidential nomination may be tightening, with a virtual three-way tie among former Vice President Joseph R. Biden Jr. and two more progressive rivals, Senators Bernie Sanders and Elizabeth Warren.
Despite the dead heat — Mr. Sanders and Ms. Warren each with 20 percent support among Democratic voters, and Mr. Biden with 19 percent — the Monmouth University Poll released Monday may hint at a concerning trend for the candidacy of Mr. Biden, the poll’s previous front-runner, whose support declined from 32 percent when the Monmouth Poll was taken in June.
Mr. Sanders, of Vermont, and Ms. Warren, of Massachusetts, both picked up several percentage points of support since June. Voters were asked whom they would support among 23 Democratic candidates.
Patrick Murray, who runs the polling institute at Monmouth, in West Long Branch, N.J., said the poll suggested that the race for the Democratic presidential nomination was heating up.
“The main takeaway from this poll is that the Democratic race has become volatile,” Mr. Murray said. “Liberal voters are starting to cast about for a candidate they can identify with. Moderate voters, who have been paying less attention, seem to be expressing doubts about Biden.”
Mr. Murray cautioned that the results were just “one snapshot from one poll.” The poll of 800 voters included 298 people considered registered Democrats or Democratic-leaning and had a margin of sampling error of plus or minus 5.7 percent within that group. It was conducted by telephone from Aug. 16 to 20.
Ms. Warren’s favorable ratings also improved, while Mr. Biden’s slipped and Mr. Sanders’s remained stable.
While only one poll, the results seemed to bolster findings of a poll released last week by the Economist and YouGov that also hinted at a tight race, showing Mr. Biden with 22 percent support, Mr. Sanders with 19 percent and Ms. Warren with 18 percent. In the poll, 72 percent of Democrats gave a “favorable” rating to Ms. Warren — two points higher than Mr. Biden’s favorable rating of 70. Mr. Sanders’s favorable rating was 65 percent.
In interviews in Iowa, some of Mr. Biden’s supporters have indicated that they lack enthusiasm for his candidacy, suggesting that his strong poll numbers may mask a softness in his support.
The Monmouth Poll found that Mr. Biden’s support had eroded in states that hold earlier primaries, dropping from 26 percent to 20 percent since June. It had also fallen among a broad range of Democrats — including those who call themselves moderate, conservative and liberal, those with college degrees and those without, white voters and voters of color.
In a statement, the Biden campaign suggested that the poll was an outlier, noting that it was based on a relatively small sample size, and that an average of national polls calculated by RealClear Politics put the former vice president ahead, with 27.2 percent to Mr Sanders’s 16.7 percent and Ms. Warren’s 16.2 percent.
Support for most of the other candidates for the Democratic nomination held firm in the Monmouth poll, albeit with far lower numbers than the three leaders.
Senator Kamala Harris of California remained steady with 8 percent support. Senator Cory Booker of New Jersey gained slightly, from 2 percent in June to 4 percent in August. Mayor Pete Buttigieg of South Bend, Ind., got 4 percent, compared with 5 percent in June.
The poll delivered bad news for another candidate, the California hedge fund billionaire Tom Steyer. Mr. Steyer entered the race in July and has said he would spend $100 million of his own money on his campaign.
Because Mr. Steyer registered less than 1 percent support in the poll, he has not yet qualified for the third televised Democratic debate in September, with a deadline to qualify looming on Wednesday.
So far only 10 candidates have qualified, meaning the debate is currently set to take place on one night.