WASHINGTON — Brian Schweitzer, the former governor of Montana, announced Saturday that he would not run for the state’s open Senate seat in 2014, a decision that further impedes Democratic efforts to retain their majority in the midterm elections.

The announcement by Mr. Schweitzer, a popular two-term governor, came as a surprise to many Democrats, who viewed him as their best hope to fill the seat being vacated by Senator Max Baucus, the Montana Democrat who earlier this year announced he would not seek re-election.

In an interview with The Associated Press, Mr. Schweitzer, 57, said that while he had considered a race, “people need to know I am not running for the United States Senate.” He said that he did not want to leave Montana for Washington.

In order to retake the Senate, Republicans would need to gain six seats. They welcomed Mr. Schweitzer’s decision.