Graham Platner (D) defeated David Costello (D) to win the Democratic primary for U.S. Senate in Maine on June 9. Platner will face incumbent Susan Collins (R) in the general election on Nov. 3.

Ahead of the primary, the New York Times wrote, "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."

Governor Janet T. Mills (D) initially ran but suspended her campaign on April 30, saying, "While I have the drive and passion, commitment and experience, and above all else — the fight — to continue on, I very simply do not have the one thing that political campaigns unfortunately require today: the financial resources." Before suspending her campaign, Mills received endorsements from U.S. Sens. Chuck Schumer (D-N.Y.) and Catherine Cortez Masto (D-Nev.) and polled second behind Platner.

Platner was 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.) 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."

Costello's experience included 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. Costello's campaign website said he ran "to help in ending the Trump Administration and GOP Congress's assault on our values, democracy, and environment."

In October 2025, multiple news outlets published stories about deleted social media posts that Platner made between 2010 and 2021 and about a since-covered tattoo on his chest that some observers said resembled a Nazi symbol. Click here to learn more.

At the time of the election, Maine used ranked-choice voting (RCV) in primary elections for certain statewide offices, and in primary and general elections for federal offices. Click here to learn more.

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 its two U.S. House seats, governor, and state legislature this year. Click here to learn more.