Senator Gary Peters of Michigan announced on Tuesday that he would not run for re-election in 2026, forcing Democrats to defend an open seat in a battleground state that President Trump won in 2024 and making the party’s already tough path to a Senate majority even tougher.

Mr. Peters, who was first elected in 2014, won re-election in 2020 by less than two percentage points. His decision not to seek a third term will set the stage for another key contest next year in a state that will already have a wide-open governor’s race, with Gov. Gretchen Whitmer departing because of term limits.

Mr. Peters had served as the chairman of the Democratic Senatorial Campaign Committee for the last two election cycles.

“I always thought there would be a time that I would step aside and pass the reins for the next generation,” Mr. Peters told The Detroit News. “I also never saw service in Congress as something you do your whole life.”

At 66, Mr. Peters is relatively young for a senator stepping down, but he had also previously served for three terms in the House. He also served in the Navy Reserve.

He released a video about his choice on Tuesday morning, saying that the “most important chapter in my life is a work in progress,” including being a new grandfather.

The path for Democrats to take back the Senate in 2026 was already steep. Republicans now hold a 53-seat majority, and with Vice President JD Vance serving as the tiebreaking vote, Democrats would need to win four seats.

The party has few pickup opportunities and is already set to defend the seat of Senator Jon Ossoff in Georgia.

Democrats successfully held a Democratic seat in Michigan last year, when Elissa Slotkin, then a Democratic House member, defeated former Representative Mike Rogers, a Republican. It was one of the most closely fought races in the nation, with Ms. Slotkin prevailing by fewer than 20,000 votes and roughly 0.3 percentage points.