A new poll suggests that the race for the 2020 Democratic presidential nomination may be tightening, with a virtual three-way tie nationally 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 Monday that the poll suggested that the race for the Democratic presidential nomination was heating up.