A month after he captured the Democratic Senate nomination in Maine with a fiery populist message, Graham Platner was facing mounting pressure to leave the race after a woman accused him of rape.

The calls for his withdrawal came in rapid-fire succession, from liberal activists who had championed his bid online, crucial early endorsers and the highest-ranking figures in his party.

By Tuesday morning, it was clear the party had turned against him. Officials from the Democratic Senate campaign arm and an aligned super PAC — the most powerful engines of the party’s infrastructure — urged him to withdraw, and top party leaders in Maine called on him to abandon his bid.

“With so much at stake, the best path forward is for Graham Platner to step aside as the Democratic nominee and address these serious allegations outside this Senate race,” said Senator Elizabeth Warren, who had campaigned with Mr. Platner and championed his bid.

Her withdrawal of support was a major blow to Mr. Platner, underscoring how even the left wing of the party was pulling away from him.

In a private call with his campaign staff on Monday evening, Mr. Platner did not announce plans to withdraw but implied such a decision would be coming, according to three people familiar with the conversation. He said that he believed he still had leverage to influence which candidate would replace him on the ticket and wanted to ensure that the movement his campaign had built would continue, the people said.

Control of the Senate could very well rest on Maine, which offers Democrats one of their best chances of winning a Republican-held seat this fall. Senator Susan Collins, the Republican incumbent, is seeking re-election in a state that former Vice President Kamala Harris won by seven percentage points in 2024.

But she is also a seasoned incumbent who has dashed Democrats’ dreams for three decades. With Republicans holding 53 seats, Democrats must defend all the seats they hold and flip four more to win control in November.

In Maine, the Democratic Party faces a hard deadline to figure out how to deal with the chaos created by Monday’s allegation: Mr. Platner has until July 13 to withdraw from the race. If he does, the state Democratic Party has until July 27 to replace him on the ticket, according to Maine state law.

It’s unclear how much leverage Mr. Platner, who has battled a series of controversies throughout his nearly yearlong campaign, may have. The leaders of the Maine Democratic Party — not national officials — are taking the lead on structuring the process for selecting a new nominee, should Mr. Platner withdraw, according to a person familiar with the discussions.

Already, Democratic activists, officials and potential candidates have begun lobbying for replacements. Several candidates are being mentioned as early possibilities, including Shenna Bellows, Maine’s secretary of state; Troy Jackson, a former president of the Maine Senate; Nirav Shah, a former director of Maine’s public health agency; and Jordan Wood, who lost a primary for a House district in northern Maine.

There’s no official process for selecting a new nominee, but top Maine Democratic officials have already begun discussing possible plans. Those could include hosting a pop-up convention or holding a statewide caucus to effectively redo the party’s primary election, according to two people who have talked with the officials and spoke on the condition of anonymity to discuss internal party conversations.

Mr. Platner built a movement-like campaign in the state, rallying Democratic primary voters around a populist economic message. But even before the rape allegation, which was reported by Politico and which Mr. Platner denies, there were signs that the steady drumbeat of controversies had hurt his candidacy.

A recent New York Times/Portland Press Herald/Siena poll found Mr. Platner locked in a neck-and-neck race with Ms. Collins. His support lagged behind the percentage of likely voters who said they would like to see Democrats control the Senate next year, a signal to some in his party that a candidate with less political baggage could be in a meaningfully stronger position.

An oysterman who has never held elected office, Mr. Platner has been dogged by controversies since he announced his bid last August, including allegations about his past treatment of women, revelations of a tattoo he had that resembled a Nazi symbol and a trove of offensive online posts he made over more than a decade.

In June, The New York Times published accounts from three women who were in romantic relationships with Mr. Platner for years. They said he could be demeaning to women and, in at least one case, even physically threatening.

One of those women, Jenny Racicot, went further in interviews with CNN and Politico on Monday, accusing him of sexual assault. She decided to go public with more detailed allegations, she told Politico, after a different woman who spoke with The Times was dismissed by some because of her conservative politics.

Concerns about Mr. Platner’s interactions with women reached the highest levels of his campaign months before the women came forward in The Times with their accounts.

Last August, Mr. Platner’s wife, Amy Gertner, told a senior campaign aide that he had been exchanging sexual messages with other women. Mr. Platner’s exchanges with women were confirmed to The Times by current and former campaign officials.

But the latest charge from Ms. Racicot appeared to be too much for many Democrats, including some who had previously been among the strongest voices dismissing concerns about his background and character.

Several hosts of “Pod Save America,” a popular liberal podcast, who testified to Mr. Platner’s “decent” character through a series of his controversies, called for him to step down. Prominent Democratic endorsers, including Senator Ruben Gallego of Arizona and Representative Ro Khanna of California raced to withdraw their support.

Planned Parenthood Action Fund, the country’s largest abortion rights organization, which had recently rallied with Mr. Platner, also rescinded its endorsement.

Others sounded vindicated.

“Graham Platner’s behavior is disqualifying (AS WE HAVE SAID THIS WHOLE DAMN TIME) and he should end his campaign,” Emily’s List, a Democratic group that backs female candidates who support abortion rights, said in a post on social media.

In recent weeks, there were signs that Mr. Platner was struggling to keep pace in the contest. He had emerged as one of his party’s most powerful online fund-raisers this election cycle, but he was being dramatically outspent by Ms. Collins and her allies, who had already poured millions into attack ads against him. His campaign and Democratic allies were set to be outspent by 2 to 1 on advertising through Election Day, according to data from the media tracking firm AdImpact.

Behind the scenes, Mr. Platner was struggling to win support from rich donors. Some worried about what damaging information from his past might next emerge, according to people familiar with private conversations.

In interviews and private meetings, Mr. Platner insisted there would be no more damaging revelations to come.