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.

In a video about his choice on Tuesday morning, Mr. Peters said, “I always knew there would come a time to pass the torch to the next generation of public servants and allow them the opportunity to bring fresh energy and ideas to our nation’s capital.”

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

In the video, Mr. Peters said that the “most important chapter in my life is a work in progress,” including being a new grandfather.