A new poll shows Joe Biden falling into a three-way tie with Senators Elizabeth Warren and Bernie Sanders, indicating a possible shift in momentum in the crowded Democratic primary field.

A Monmouth University poll of Democratic voters nationwide released Monday found Warren, Sanders and Biden in the top tier of candidates, but Biden's support dropped significantly from previous polls by the New Jersey-based institute and other groups. The poll, with a sample size of 298 registered Democratic or Democratic-leaning voters, showed Warren and Sanders with 20% support, and Biden at 19%.

Warren and Sanders have been vying to present themselves as the standard-bearers of the Democratic Party's progressive wing through bold proposals and grassroots campaigning. The poll showed a significant drop in support for Biden's well-funded campaign and his more moderate, pragmatic politics, which he has argued are the best suited to defeat President Trump in 2020.

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The results show Biden's support dropped 13 points since the last Monmouth poll in June. Warren and Sanders gained 5 and 6 percentage points, respectively.

Senators Bernie Sanders and Elizabeth Warren shake hands before the start of the first night of the second 2020 Democratic debate in Detroit, Michigan, on July 30, 2019. LUCAS JACKSON / REUTERS

Behind the three top candidates were Kamala Harris (8%), Cory Booker (4%), Pete Buttigieg (4%), Andrew Yang (3%), Julián Castro (2%), Beto O'Rourke (2%) and Marianne Williamson (2%). The rest of the candidates polled at 1% or lower.

Monmouth's pollsters attributed the decline in support for Biden's candidacy to several developments. The former vice president lost the backing of some voters who identified as moderate and conservative Democrats, with Warren and Sanders gaining traction among these voters. Biden also lost support among more liberal Democrats, who contributed to the modest gains by candidates like Booker, Castro, Williamson and Yang.

The Monmouth poll was conducted between August 16 to 20 and had a margin of error of 5.7%.