The former governor was recently elected board chairman of Stillwater Mining Co., Montana's largest publicly trading company and said he is enjoying his life.
"I have responsibilities here in Montana, my family first. I have taken on a new life at the Stillwater mine. I owe it to the 1,670 people who work at the Stillwater mine that we continue to manage it and make it the best place to work in Montana," Schweitzer said. "This is my home, not Washington, D.C."
Schweitzer said recent criticism over politically active nonprofits connected to him had no bearing on the decision and said such criticism isn't new.
"This isn't my first rodeo," Schweitzer said.
Montana's open Senate seat is one of several being targeted by Republicans who hope to regain Senate control in the 2014 elections. Republicans need to pick up six seats to win back the majority and enjoy several advantages: the GOP is defending fewer incumbents than Democrats and could benefit from the fact that the party controlling the White House usually loses seats during the midterm election of a second-term president.
Democrats need to defend 21 seats, including seven in largely rural states that Republican Mitt Romney carried in 2012. Republicans hope to unseat four key incumbents: Mark Pryor of Arkansas, Mark Begich of Alaska, Kay Hagan of North Carolina and Mary Landrieu of Louisiana.
The brash, jeans-wearing Schweitzer last year told the AP: "I am not goofy enough to be in the House, and I'm not senile enough to be in the Senate."
The 57-year-old Democrat left office in January after eight years in office with high approval ratings, but he wasn't able to run again because of term limits. The unconventional governor, who easily won re-election in 2008, always displayed a feel for tapping into Montana's conservative-leaning yet libertarian politics.
In Helena, his heavy-handed style proved adept at largely getting his way with the state budget despite fostering a confrontational and sour relationship with majority Republicans. He often touted the state's surpluses at a time when many others were floundering.
The outspoken governor never missed an opportunity to leave a larger-than-life impression. He once stormed New York's Times Square with a bullhorn handing out Montana-made promotional trinkets from a semi-truck.
But Schweitzer said he is enjoying pursuits other than politics, with a new lake house and a small ranch in the mountains.
"I don't want a job where I have to wear a suit, and my dog isn't welcome," he said.
Other Democrats who expressed an interest in running, including State Auditor Monica Lindeen and schools Superintendent Denise Juneau, had been waiting on Schweitzer's decision.
The announcement surprised a Democratic Party that was meeting Saturday in Lewistown for its annual convention, and attention quickly turned to other potential candidates.
"Democrats were ready to get behind him in a bid for U.S. Senate," said Juneau. "Since his announcement, I have been receiving encouragement to run, and I plan to give careful thought to that decision."
State Sen. Kendall Van Dyk of Billings, said he understood the decision to spurn a dysfunctional Congress. "He chose Montana trout over the rattlesnakes," Van Dyk said.
However, he added, Schweitzer "has done tremendous things for the state, and any Democrat will tell you he has done tremendous things for the party. Montana needs Brian Schweitzer. And I believe this isn't the last we have heard of him."
Republicans are hopeful that freshman U.S. Rep. Steve Daines will run for the open seat. Some Republicans are also advocating former Montana Gov. Marc Racicot, who served from 1993 to 2001 and later chaired the Republican National Committee.
Daines said Saturday that he will continue to consider the race.
"My focus is fixed on serving the people of Montana and doing the job they sent me to do," he said in a statement. "I will continue to give this decision the consideration it deserves, and am still taking time to talk with my family and the people of Montana about how I can best be of service to our state."
As Schweitzer was mulling his options, the Montana Republican Party circulated a 2010 Internal Revenue Service form that showed a politically active nonprofit group called The Council for a Sustainable America used the same Helena post office box used for Schweitzer's 2008 re-election campaign and was signed by David Gallik, the state commissioner of political practices appointed by Schweitzer at the time.
The Council for a Sustainable America spent about $60,000 on politics in 2010, but it did not say in which races.
Schweitzer adviser Franklin Hall said at the time the nonprofit was never used for Montana politics or to aid Schweitzer.
Baucus, 71, announced in April that he was retiring. He was elected to the Senate in 1978 after serving two terms in the House.