When U.S. Sen. Susan Collins defied the polls and shocked political observers in 2020, moderates were her secret weapon.

After trailing in public polling before Election Day, Collins won by 9 percentage points. At the same time, Democrat Joe Biden won the presidential race here by the same margin — meaning Collins overperformed the top of the ticket by 18 points.

That means untold thousands of Mainers voted for both Collins and Biden in 2020.

In 2026, those voters are facing an unprecedented choice: Stick with Collins, or elect political newcomer Graham Platner, a 41-year-old oyster farmer and combat veteran who is running on a progressive, populist platform.

Whom they choose could well determine the outcome of the election, experts say. And there’s evidence that both candidates have some work to do to win that bloc over.

Collins sports low approval ratings, and some early surveys show Platner beating her in a head-to-head matchup.

Platner, meanwhile, has to find a way to win over those skeptical of his progressive politics, including some diehard supporters of Gov. Janet Mills, 78, who suspended her Senate campaign last week.

Mills’ name will still appear on the primary ballot in June, and some supporters say they plan to vote for her in the primary. Some even say they’ll vote for her as a write-in in the general election.

“There are no circumstances where I would vote for Graham Platner,” said Lynn Bromley, a former state lawmaker from South Portland who appeared in Mills’ ad attacking Platner’s past comments on sexual assault.

Bromley said she saw rampant sexism in the Democratic primary, where Mills was frequently criticized for her age. She said Mills got no credit for her experience advancing Democratic causes, all while voters backed someone with no political experience and a history full of material to be used in Republican attack ads.

“It felt like Hillary (Clinton) burning all over again,” Bromley said, referring to the bruising 2016 presidential primary and general elections.

Carol Eisenberg, a Mills supporter from Portland, said she too has concerns about Platner. She plans to vote for Mills in the primary.

But Eisenberg is planning to vote for Platner in the general election because her highest priority is unseating Collins.

“She’s been a dangerous force, and a powerful one,” Eisenberg said.

Collins’ pitch to moderates has centered on her constituent services and her ability to bring home federal dollars for community needs. Ads supporting her feature the fire station improvements she helped push for in D.C., and she often holds events touting the money she’s won for the state.

Over the last five years, Collins, who is the first Mainer to chair the Senate Appropriations Committee in a century, has secured over $1.5 billion in federal earmarks for more than 670 projects, her office said.

“Centrist voters are going to have a clear choice this fall because Graham Platner has made it clear that he doesn’t think that policy should come from the center, it should come from the far left wing,” Collins’ campaign spokesman Shawn Roderick said. “Her likely opponent, who is perhaps the most extreme Senate candidate in the country, repeatedly scoffs at moderates and people who believe in working together to get things done.”

Platner’s campaign says their candidate plans to continue holding town halls, where he will talk about the outsized influence of billionaires and corporations in politics. His pitch to moderates is about tying Collins to Trump’s unpopular policies, including the president’s war with Iran.

There are signs his message is connecting. In a March poll by Emerson College, Platner was the only candidate above water among Independents with a plus-5 point net favorability rating, compared to Collins at minus-29 points.

“Every time Sen. Collins leverages a little bit of an earmark to build a bridge or a road, that just does not make up for the fact that Mainers right now are hemorrhaging money left and right because she has backed the Trump administration’s agenda to bleed people dry,” campaign manager Ben Chin told reporters last week.

However, the same poll that showed Platner performing well with Independents showed that 20% of respondents didn’t know enough about the candidate to say whether they liked him.

James Melcher, a political science professor at the University of Maine Farmington, said that Collins could benefit from older women who are put off by Platner’s history.

At the same time, Melcher said Platner could peel some Republicans from Collins, especially younger men, who supported Trump in previous elections, because of his blunt style and calls to shake up the system.

“I think there’s going to be more split-ticket voting with this matchup than there would have been with Janet Mills as a more traditional candidate,” Melcher said. “I don’t know who comes out ahead in that, though.”

Once the wounds of the Democratic primary heal, Melcher said the deciding factor in the general election will come down to which party voters want to see in control of the U.S. Senate.

“I think the overwhelming majority of people who would have voted for Janet Mills will be willing to vote for Platner in the end,” Melcher said.

Ella Mik’aella Bowman, of Hinkley, said she didn’t know whom to support in the Democratic senate primary, and was looking forward to seeing Platner and Mills debate.

Now that Mills has suspended her campaign, Bowman plans to throw all of her support behind Platner.

“I’m not a supporter of the (Trump) movement and will vote in support of anyone who can help dig our country out of this dark and corrupt time in our history,” Bowman said.