"We all know it's going to take a sense of duty to stand up for what's right. The idea of selfless service, to put our country on the right track again, and the courage to tackle the national debt and defend those who don't have a voice: our seniors, our veterans, our children," he says in an announcement video touting his military experience. "I'm ready to do what it takes to put the people of the great state of Montana first."

Democrats have been scrambling for a candidate ever since Sen. Max Baucus (D-Mont.) shocked many with a retirement announcement earlier this year, and former Montana Gov. Brian Schweitzer (D) decided against a run. A number of other high-profile Democrats have turned down a bid, and Walsh has become the consensus favorite for the party since Schweitzer's decision.

While Walsh doesn't have much political experience — his run for lieutenant governor last year was his first campaign of his career — on paper he looks like a candidate who might be able to keep the race competitive.

Republicans haven't officially landed a top recruit of their own, but freshman Rep. Steve Daines (R-Mont.) is widely expected to run.

Montana is a must-win state for the GOP to pick up the six seats it needs for Senate control. The GOP feels bullish about its chances with Daines, who has been raising money at a fast clip.

The GOP didn't waste any time in attacking Walsh, blasting him for problems at the Montana Department of Military Affairs while he was running the department and accusing him of sexism because the department was sued for unfair hiring practices toward women.

"Democrats beat their chest about a war on women but actively recruit candidates like John Walsh, who has an atrocious record of defending women from harassment and degradation," says National Republican Senatorial Committee spokeswoman Brook Hougesen. "Folks in Montana yearn for leaders who speak the truth and will be a voice for them, not just another man who says one thing and does another."