Three candidates are running in the Democratic primary for U.S. Senate in Maine on June 9. As of April 2026, Janet T. Mills (D) and Graham Platner (D) lead in media attention, fundraising, polling, and endorsements. The winner of the primary will face incumbent Susan Collins (R) in the general election.
According to the New York Times, "The stakes of the race, which has quickly become one of the most contentious Democratic primaries of the midterm season, could not be higher for the party. To have any chance of retaking the Senate, Democrats must defeat Ms. Collins, a Republican who is widely seen as vulnerable but who has repeatedly dashed their hopes of unseating her. As they look toward the general election, Maine Democrats are debating whether it is safer to nominate an experienced, older governor who struggles to energize the party’s base, or a younger insurgent who is feeding off the country’s anti-Washington energy but appears to have significant political baggage."
Mills was elected governor of Maine in 2018. Mills was a member of the Maine House of Representatives from 2002-2009 and the state's attorney general from 2009 to 2011 and again from 2013 to 2019. U.S. Sen. Chuck Schumer (D-N.Y.), U.S. Sen. Catherine Cortez Masto (D-Nev.), Michigan Gov. Gretchen Whitmer, and Maine Attorney General Aaron Frey (D) have endorsed Mills. Her campaign website said, "Janet doesn’t just fight, she wins for Maine: protecting our health care when politicians try to rip it away, taking on the drug companies fueling the opioid crisis, and defending our state from Donald Trump’s attacks."
Platner is a Marine and U.S. Army veteran, and oyster farmer. U.S. Sens. Bernie Sanders (I-Vt.), Ruben Gallego (D-Ariz.), and Elizabeth Warren (D-Mass.) have endorsed Platner. Platner's campaign website said, "I view most of my job as a US Senator as to do two things: one, to ban billionaires buying elections; two, to dismantle the 'billionaire economy', in favor of an economy that works for the American worker, for small business, for the vast majority of Americans."
Both Mills and Platner have said that they would be the best suited to challenge Collins in the general election. Mills has said that Republicans “will tear [Platner] apart if he’s the nominee” and has run campaign ads criticizing Platner's past social media comments about sexual assault. Platner has said that “it is far more risky to go with someone who’s been in politics for a very long time.” When Mills first launched her campaign, Platner released an ad in which a woman says, "Janet Mills, again? She was a good governor, but I think it's time for a change."
In October 2025, multiple news outlets published stories including deleted social media posts that Platner made between 2010 and 2021. In the posts, Platner wrote about multiple topics, including his political views, the police, the military, and sexual assault. In response, Platner apologized for the posts and said that he had made them after returning from his deployments in Iraq and Afghanistan and that he was struggling with PTSD and depression. Also in October 2025, Platner appeared on an episode of the podcast Pod Save America and responded to a video of himself, in which a tattoo on his chest was visible that some observers said resembled a Nazi symbol. Platner said he was unaware of the resemblance and later posted a video showing that he had gotten the tattoo covered up. Click here to learn more.
David Costello (D) is also running in the primary. His experience includes working as an environmental policy consultant. Costello also ran for Senate in 2024. In that election, he placed third of four candidates and earned 10.8% of the vote statewide.
Maine uses Ranked-choice voting (RCV) in primary and general elections for federal and state offices. Click here to learn more.
As of April 2026, the Cook Political Report and Sabato's Crystal Ball rated the general election a Toss-up while Inside Elections rated it Tilt Republican.
Thirty-three of the 100 U.S. Senate seats are up for election this year, and another two seats are up for special election. Democrats hold 13 of the seats up for election, and Republicans hold 22. The Senate seat in Maine is the only one that Republicans are defending in a state that Kamala Harris (D) won in the 2024 presidential election. Democrats are defending two seats in states that Donald Trump (R) won in the 2024 presidential election: Georgia and Michigan.
Heading into the 2026 elections, Republicans held a 53-45 majority in the chamber and both independent U.S. senators — Sanders and Angus King (I-Maine) — caucus with the Democrats.
Maine is also holding elections for it's two U.S. House seats, governor, and state legislature this year. Click here to learn more.