Graham Platner, the progressive oyster farmer who toppled Maine’s political establishment even as a series of unsettling revelations about his past rattled his party, won the Democratic nomination for Senate on Tuesday, according to The Associated Press.

Mr. Platner’s victory, long expected after Gov. Janet Mills suspended her campaign in April, puts him in a general-election contest against Senator Susan Collins, a five-term Republican with a history of frustrating Democratic attempts to oust her.

In a victory speech that took aim at Ms. Collins, Mr. Platner declared that his political movement would “take back our power” in November and subsequently enact a raft of progressive legislation, including codifying abortion rights and passing a single-payer health care system known as Medicare for all.

Mr. Platner dismissed the news reports about his past as immaterial to the coming general election.

“In trying to so hard to understand me, they failed to understand that this is not about me at all,” he said. “This is a movement about us, about the far too many working far too hard and struggling far too much.”

The Collins-Platner contest is expected to be one of the most expensive, hardest-fought Senate races in the country, and the stakes could hardly be higher. Maine is the only state with a Republican-held Senate seat on the ballot this year where President Trump lost in 2024. Democrats must flip at least four Republican-held Senate seats in November to win a majority in the chamber.

Mr. Platner, 41, finished well ahead of the other two Democrats on the Senate ballot in Maine: Ms. Mills, who has not endorsed Mr. Platner and last week went out of her way to remind Mainers she was still on the ballot, and David Costello, a former state government official in Maryland who attracted few donors and little attention. With 37 percent of votes counted on Tuesday night, he was leading Ms. Mills by about 72 percent to 19 percent, with Mr. Costello drawing 8 percent of the vote.

Mr. Platner’s primary victory caps a remarkable rise for a political neophyte whose only prior public service was as harbor master in his small hometown, Sullivan. Endorsed by Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, Mr. Platner has used his plain-spokenness and working-class image to become a tribune of the progressive left at a moment when Democrats are jockeying over the future direction of the party.

There was little drama in Mr. Platner’s victory in the Maine primary, but that has not been the case in recent weeks for his campaign.

He has faced disclosures about offensive old posts he made on social media, for which he has since apologized, as well as a tattoo he had that is widely recognized as a Nazi symbol. In recent weeks, Democratic politics was rocked by a New York Times report detailing that women he had dated recounted him acting in disturbing ways, as well as reports that he had sent sexually explicit text messages to women outside his marriage.

A combat veteran of the Iraq war, Mr. Platner has spoken openly about his struggles with PTSD, depression and drinking after leaving the military. While Ms. Mills only briefly used his past statements against him in paid advertising, Ms. Collins and her Republican allies are likely to spend tens of millions of dollars to try to use his past to incinerate his reputation in Maine.

The principal super PAC that backs Republican Senate candidates has already reserved $42 million in advertising for the Maine race. Its Democratic counterpart is in for $24 million. Both numbers are likely to increase substantially, and other groups are expected to buy time in the state, too.

Now that he has won the primary, Mr. Platner faces the task of uniting a Democratic Party that is divided over his candidacy and debating whether it remains a worthwhile investment politically and financially to rally behind a candidate with such a troubled past.

Last week, Democratic officials and candidates from coast to coast found themselves peppered with questions about how they could still support Mr. Platner. Some, like Representative Madeleine Dean of Pennsylvania, declared Mr. Platner to have disqualified himself from office, while Mr. Sanders and other progressives stuck by him.

So far, the Senate Democrats’ campaign arm, which had backed Ms. Mills before she dropped out, has offered only token assistance to Mr. Platner’s campaign. The coming general election will test the extent to which that group is willing to commit resources and personnel to a candidate it has long preferred not to be the party’s nominee.