Democratic U.S. Senate candidate Graham Platner speaks during a primary election night watch party after winning the Democratic nomination Tuesday in Blue Hill. (Robert F. Bukaty/Associated Press)

Political newcomer Graham Platner clinched the Democratic nomination for U.S. Senate on Tuesday, setting up a showdown with five-term Republican incumbent Susan Collins.

Platner looked to have picked up more than 72% of the vote with 93% of the votes counted. The Associated Press called the race for Platner shortly after 9:20 p.m. Tuesday.

The primary was effectively a formality after Gov. Janet Mills suspended her campaign in late April.

Still, Tuesday was Platner’s first test before voters, and he passed.

“You have built the most powerful grassroots movement in the history of this state. This is because of you. And when we finally defeat Susan Collins, that will be because of you, too,” Platner told supporters at his watch party in Blue Hill, the small town where he was born.

He stood onstage with supporters standing behind him holding signs that read, “A Senator money can’t buy,” and “They don’t know Maine.”

Mills’ name remained on the ballot, a point she reminded voters of last week, but she captured only about one in five votes. Long-shot David Costello, a former government official from Brunswick who ran unsuccessfully for Senate two years ago, was also in the race, netting some 8% of the votes.

Andrea LaFlamme, an adjunct professor and labor leader, mounted a write-in campaign, but she placed a distant fourth with less than 1% of the vote.

Platner, a 41-year-old oyster farmer and combat veteran from Sullivan, will face Collins, 73, this fall in what is expected to be one of most closely watched races in the country.

Democratic U.S. Senate candidate Graham Platner, right, and his wife, Amy Gertner, gesture to supporters during a primary election night watch party Tuesday in Blue Hill. (Robert F. Bukaty/Associated Press)

Democrats, who hope to take control of the Senate, view Maine as one of their best pickup opportunities. Collins is the only Republican senator running in a state won by Democrats in the 2024 presidential election.

Platner thundered onto the political scene last summer when he gained the backing of independent Sen. Bernie Sanders of Vermont, a progressive icon with a lot of support in Maine.

He easily beat back a challenge from the 78-year-old Mills, who was recruited by Senate Minority Leader Chuck Schumer, D-N.Y.

Mills didn’t enter the race until last fall, and could not blunt the momentum of Platner’s populist, anti-establishment movement. She was trailing badly in public polling and fundraising when she suspended her campaign. Termed out of the governor’s office, Mills will become a private citizen next year.

Still, some voters checked her name off on their ballots, not ready to switch to Platner just yet.

Mary Shea, 65, of Falmouth, said she cast a protest vote for Mills, with some hesitation. “She’s not the right person for the moment, but I don’t feel good about Platner,” Shea said on Tuesday.  

Shea, however, said she would vote for Platner if he was the nominee in November.

“Collins has to go,” she said.

In Auburn, Marion Holmes felt leery about Platner. She said the recent accusations about his past interactions with women and some of his old social media posts are troubling.

She said she ranked Mills first, then Platner. She plans to support him in the general election, but said she doesn’t want to send another politician to Washington who is going to treat women badly, saying President Donald Trump has done enough of that.

“I don’t want somebody in Washington embarrassing me,” she said. “I don’t want Susan Collins either, but I don’t want him (Platner) doing that. I’d rather have Mills if he’s going to do that.”

In his victory speech, Platner took a moment to thank Mills for what he said was her lifetime of public service. And he said he would work to earn the trust of her supporters in November.

In a statement Tuesday, Mills once again notably avoided endorsing Platner. Instead, she touted her accomplishments as governor and thanked voters for their support.

“I am grateful to Maine people and incredibly proud of what we have accomplished together,” Mills said. “I will continue to fight with everything I have to improve the lives and livelihoods of Maine people.”

Now, the race moves to the general election, where independent voters are the state’s second-largest voting bloc behind Democrats, comprising nearly a third of the electorate.

In an early salvo aimed at winning over general election voters, Platner dedicated much of his election night speech to criticizing Collins. He bashed her for voting for Brett Kavanaugh’s Supreme Court confirmation, and for growing her wealth while serving in the Senate. And he criticized her for taking campaign donations from private equity, which he said is ruining various institutions beloved by Mainers, including the Boston Red Sox.

“Susan Collins is getting rich while we’re getting screwed,” Platner said.

In response to the criticism, Collins’ campaign said she has a track record of bipartisan accomplishment for Mainers.

“”Mainers aren’t looking for bitter campaigns, grand promises, or angry speeches riddled with lies. They’re looking for results. They want affordable healthcare, safe communities, good-paying jobs, strong schools, and someone who will show up and do the work,” the statement from spokesperson Shawn Roderick read in part. “Maine people are practical. They care about whether their communities are stronger and their families are better off. That’s exactly what Susan Collins is focused on every single day.”

Platner’s campaign has been rocked by controversies almost from the start, including revelations of offensive past internet posts, Platner’s disclosure that he had a tattoo of a Nazi symbol and the recent reporting on his personal life.

Republicans are all but certain to remind voters of those scandals.

“Platner is easily the most toxic candidate of the cycle and the fact that Democrats have embraced him in service of a radical socialist agenda has placed the final nail in the coffin of their chances to win Maine in November,” Republican National Committee Chairman Joe Gruters said in a written statement.

But on Tuesday, Maine Democrats stuck with their senate nominee.

“Everyone makes mistakes in life. I understand his struggles, and I give him grace,” said Louise Frank, 67, of Woodstock. “I think he’s gruff. I think he’s kind of messy, but I think he’s honest. Something’s got to change, and I have hope that he will do it.”

Staff Writers Sophie Burchell, Kendra Caruso and Rose Lincoln contributed to this story.