The progressive senator Bernie Sanders has called on Graham Platner to withdraw from the US Senate race in Maine, citing “very serious allegations” of sexual assault.

While Platner has denied the claim, first reported by Politico, a growing list of Democrats have urged him to stand aside as the party’s candidate. He has said he is “taking the time to reflect on the best path forward” in the wake of the story.

Sanders, one of Platner’s earliest and most influential supporters, is the latest to call on him to withdraw.

Earlier on Tuesday, Zohran Mamdani, New York City’s mayor and another high-profile figure on the progressive wing of the Democratic party, said the “only appropriate response” in the accusation against Platner was to end his campaign. “I believe that it’s time for him to drop out of the race,” Mamdani told reporters.

In the Politico report, published on Monday, Jenny Racicot, 41, who previously dated Platner, said he forced her to have sex despite repeated objections. Platner denied her claims in a statement to Politico.

Institutional support collapsed rapidly, however. Chuck Schumer, the US Senate minority leader, and Kirsten Gillibrand, who chairs the party’s Senate campaign arm, called for Platner’s withdrawal and said the committee would not fund his campaign if he stayed on the ballot.

Maine’s state party leadership has also urged him to step down. Senator Elizabeth Warren, previously a vocal backer, asked him to withdraw, while Ruben Gallego and Ro Khanna, the senator and representative who had both campaigned with Platner, also rescinded their endorsements.

Platner, 41, entered the race last August virtually unknown: a Marine Corps combat veteran turned Maine oyster farmer whose viral, anti-establishment campaign overtook state governor Janet Mills – the party establishment’s preferred candidate – before she withdrew.

He went on to win the primary by the largest margin in state history, making Maine one of the Democrats’ best hopes of flipping a Senate seat this cycle.