The popular Democrat, who left the governor's office in 2013 after two terms, told the Associated Press that he “kicked the tires” on the race but decided he preferred Montana to Washington.
“There are all kinds of people that think I should be in the U.S. Senate. But I never wanted to be in the U.S. Senate,” he said.
Schweitzer's decision will disappoint Democrats in Washington and in Montana who saw him as their best chance to hold the seat now held by Max Baucus, who also surprised the party this year by announcing he wouldn't seek a seventh term.
The Democratic Senatorial Campaign Committee just Friday had touted a new poll showing Schweitzer with strong public approval numbers. A prominent national progressive group also had organized a “Draft Schweitzer” campaign to encourage him to run.
Montana Democrats are holding their annual state convention this weekend. Sen. Jon Tester told the Missoulian newspaper Friday that he thought Schweitzer would still run, but said he needed to “hurry up” with a public announcement.
But Schweitzer, a populist Democrat known for wearing jeans and a bolo tie, had made no secret of his interest in staying in Montana.
He told the AP Saturday: “I don't want a job where I have to wear a suit and my dog isn't welcome.”
Democrats are facing a difficult political map in 2014. They must defend at least 20 of the 35 Senate seats on the ballot, including five in which the incumbent is retiring.
Montana is one of seven seats on the ballot next year held by a Democrat that was carried in 2012 by Republican presidential nominee Mitt Romney.
Republicans are aggressively targeting those and other Democratic-held seats; Democrats have few obvious targets among the current Republican-held seats.
Pending the outcome of an October special election in New Jersey where Democrats are favored, Republicans will need to win a net of six Senate seats to regain the Senate majority for the first time since 2007.
Guy Cecil, executive director of the Democratic Senate Campaign Committee, said in a statement that the party was still confident it could win in Montana next year, and that “the overall math still favors Democrats.”
Referring to Sen. Mitch McConnell of Kentucky, he added, “Our incumbents are positioned to win, we've already recruited a strong challenger to Mitch McConnell and Republicans have failed to expand the Senate map into blue and purple states.”