It’s official: Republican Sen. Susan Collins will face Democrat Graham Platner this fall, NBC News projects, in what will be a marquee election in the fight for control of the Senate.

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Collins and Platner both won their primaries on Tuesday, in a predictable result. Collins, first elected to the Senate in 1996, ran unopposed for re-nomination as she seeks a sixth six-year term.

And Platner, a veteran and oyster farmer running in his first political race, faced little Democratic competition as two-term Gov. Janet Mills suspended her campaign after failing to gain traction in the race. She still appeared on the primary ballot.

While the primary results were foreseeable, what happens next is anything but. The Senate election has already become a battleground over the future of the Democratic Party and what voters think is most important, as Platner faces numerous controversies about his past conduct.

And that’s before the real campaigning between the resilient incumbent and the brash outsider has even kicked off.

Maine looms large in the battle for the Senate majority. Democrats currently hold 47 seats and need a net gain of four to capture the majority. Collins, as the only Republican senator from a state President Donald Trump lost in 2024, is an inviting target for defeat.

But Collins has proven to be a political survivor, repeatedly defying attempts to oust her — and hoping to do so yet again in a difficult environment.

In 2020, when Democrats were brimming with optimism about winning the Maine Senate seat, Collins over-performed the polls and won re-election by more than 8 points, even as Trump lost the state by about 9 points.

Collins is backed by the president, who said in March on Fox News, “I hope she wins. Because we have to. She’s a good person, actually. But we have to win. We have to keep the majority. Otherwise all of the things that we’ve done are going to go down the tubes.”

In previous election cycles, Democrats have nominated center-left figures who were backed by the party establishment and fell short against Collins. This year, Platner is unlike any opponent Collins has faced: a populist outsider who has built a connection to supporters with calls to upend the status quo, tax the rich, expand the safety net and get money out of politics.

But he also brings significant baggage and controversy.

There’s the chest tattoo that Platner recently had covered after saying he didn’t know it had a Nazi association. There are his years-old Reddit posts criticizing police, suggesting some victims of sexual assault bore some responsibility and labeling himself a “communist.” He has chalked it up to “Internet s—posting” from an earlier era that doesn’t reflect his views today.

And there are recent allegations by ex-girlfriends about “toxic” or “unsettling” behavior, after which Platner denied there was “physicality,” as alleged by one woman.

The checkered past has prompted unease among Democratic leaders in Washington, some of whom endorsed Platner after Mills dropped out but have recently sought to avoid talking about him. Some figures in the party say he should drop out. Party insiders could replace him, but only if he willingly quits the race before July 13 — and Platner has insisted he won’t drop out.

Meanwhile, his most prominent backers in the Senate — most notably Sen. Bernie Sanders, I-Vt., and Sen. Elizabeth Warren, D-Mass. — are standing by him. Recent general election polls taken before and after his latest controversies show a competitive race with Collins.

Collins, for her part, has sought to downplay her party affiliation and national dynamics and instead localize the race, focusing on money she has brought home to Maine using her perch as chair of the powerful Senate Appropriations Committee. Last week she cast her 10,000th consecutive vote in the chamber.