Sen. Jon Tester, D-Mont., said he learned of the retirement plans on Monday. He said Baucus told him he wanted to return to Montana, and noted that if he waited until the end of his next term he would be nearly 80.
Baucus, in the interview with the AP, said: "Been here 40 years. No regrets. It is time to do something different."
Maneuvering began almost instantly for the 2014 race.
"The opportunity to try and get the country moving again like we did in Montana, that's appealing," said Schweitzer, who outpolled Baucus in a hypothetical matchup in the recent poll. "I'm a fixer."
Possible Republican candidates include former Gov. Marc Racicot; former Rep. Denny Rehberg, who lost to Baucus in 1996 and to Tester last fall; former Rep. Rick Hill and Rep. Steve Daines. State Sen. Champ Edmunds of Missoula and former state Sen. Corey Stapleton, had already announced they would run against Baucus.
"Montana is a state where Republicans can and will do well," said Sen. Jerry Moran of Kansas, the GOP campaign committee chairman, pledging to provide the resources needed to turn the seat Republican.
The state twice voted against Obama in presidential races. Despite the president's presence on the ticket in 2012, Tester won a second term in a hotly contested challenge, and another Democrat, Steve Bullock, was elected governor.
Sen. Michael Bennet, D-Colo., who heads the Democratic Senatorial Campaign Committee, touted last year's re-election of Tester and said, "We will continue to invest all the resources necessary to hold this seat."
Democrats will be defending 21 seats next year, compared with 14 for Republicans.
Baucus joined Jay Rockefeller of West Virginia, Frank Lautenberg of New Jersey, Tim Johnson of South Dakota, Tom Harkin of Iowa and Carl Levin of Michigan in announcing his retirement plans.
Republicans Saxby Chambliss of Georgia and Mike Johanns of Nebraska also have decided not to seek re-election next year.