Walsh told supporters Thursday morning that he will run for the office that has been the focus of a great deal of speculation since Sen. Max Baucus announced this year that he will retire at the end of 2014.
The 52-year-old former Montana National Guard commander enters the race with just one election under his belt. He won a tight race last year as Gov. Steve Bullock's running mate.
He said the Senate race will be one of the biggest challenges of his life, but he expects he can win.
"Washington is broken, so we need to take a look at what is happening back there and come together. The people need to work together to solve a problem, and that is not what is happening," Walsh said. "I think Washington, D.C., needs someone fresh who isn't a career politician to go back there and represent the citizens of Montana."
The up-in-the-air Montana race could help determine control of the Senate.
Republicans, who like their chances in the GOP-leaning state, need to pick up six seats to recapture the Senate majority and are trying to take advantage of geography and history in their quest.
Democrats must defend 21 seats, including seven in largely rural states that President Barack Obama lost in 2012, and the party that controls the White House typically loses seats during the midterm elections of a second-term president.
Baucus shocked Montana politics when he announced in April he would not seek a seventh term despite a large campaign war chest and no serious announced opposition. Baucus, who in recent years has bemoaned increased polarization in Washington, said he was walking away from chairmanship of the Senate Finance Committee and high-ranking seniority so that he could enjoy a life outside of politics for the first time since the early 1970s.
The announcement created a scramble for candidates on both sides, and a free-for-all for the first open Senate seat in the state since 1978.
Several statewide elected Democrats -- and the man once assumed to be the front-runner in the 2014 race, former Gov. Brian Schweitzer -- have spurned recruitment efforts.
Former Lt. Gov. John Bohlinger -- a Republican turned Democrat -- has said he is considering it. And a political unknown and former San Francisco banker, Dirk Adams of Wilsall, has said he will mount a campaign.
Many Republicans expect U.S. Rep. Steve Daines will enter the race as their front-runner. Daines has been raising money consistent with a Senate run and has said he could announce his decision fairly soon.
Former Gov. Marc Racicot, who used to chair the Republican National Committee, rejected a run of his and encouraged Daines to enter the race.
"Six or seven prominent Democrats have already said no to this race, and Lieutenant Governor Walsh is their last hope for a recruit. We see opportunity here," said Montana Republican Party executive director Bowen Greenwood. "Republicans are encouraging Steve Daines to run, and I think he'd be the best candidate available on either side of the aisle."
Two other low-profile Republicans have already said they are running, current state Rep. Champ Edmunds of Missoula and Kalispell air traffic manager David Leaser.
Bullock congratulated his lieutenant governor in a statement, saying that "John will put politics aside and get things done."
Walsh, the former adjutant general who has said he never considered a run before Schweitzer bowed out, predicted his likely opponent -- Daines -- could have trouble because of his role in a Congress that shut down the government amid stalemate.
"I don't know Congressman Daines real well. I think he is a nice man. But I believe he is part of broken system back there," Walsh said. "I am a little disappointed that he hasn't stepped up to come up with a solution to the problem."
Daines' spokeswoman Alee Lockman said the Republican is "working toward real solutions that fix Washington and get our country back on track -- not politics."
Walsh -- who has two grown children with his wife, Janet, and a grandchild -- said he is a Democrat owing to his roots in the union stronghold of Butte.
"My dad was a pipefitter and was a strong labor man," Walsh said. "I was raised with the Democratic values, Montana Democratic values and I have carried those values on throughout my lifetime."
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