"I don't want to die here with my boots on. There is life beyond Congress," the 71-year-old Baucus said in a telephone interview with The Associated Press.
He became the eighth senator to announce retirement plans for 2014, and the sixth Democrat. One public poll recently suggested he would have faced a difficult challenge if he had sought a seventh term.
Republicans must gain six seats in 2014 to win a majority, and they said the retirement enhanced their prospects.
Yet Democrats were cheered when former Democratic Gov. Brian Schweitzer, who recently stepped down after two terms, swiftly expressed interest in the race.
In a brief statement, President Barack Obama said Baucus "has been a leader on a broad range of issues that touch the lives of Americans across the country."
Sen. Chuck Grassley, an Iowa Republican and Baucus' frequent legislative partner, was complimentary, too. "We ran the Finance Committee for 10 years together, and every bill except for three or four was bipartisan," he said in a statement. "The Senate will be worse off as a deliberative body when Senator Baucus leaves."
In a written statement, Baucus sketched an ambitious agenda for the rest of his term, topped by an overhaul of the tax code.
"Our country and our state face enormous challenges - rising debt, a dysfunctional tax code, threats to our outdoor heritage and the need for more good-paying jobs," he said, adding several Montana-specific priorities as well.
Baucus, a fifth-generation Montanan, was elected to the Senate in 1978 after two terms in the House. He became the top Democrat on the Finance Committee in early 2001. He has held the position ever since on the panel — which has jurisdiction over taxes, Medicare, Medicaid, health care and trade — as chairman when his party held a majority and as senior member of the minority when Republicans were in power.
The panel has a long tradition of bipartisanship, but Baucus ascended to power in an era of increasing partisanship in Congress.
Many Democrats were unhappy when he worked with Republicans to enact the tax cuts that President George W. Bush won in 2001. And then again in 2004 when Congress pushed through a GOP plan to create a new prescription drug benefit under Medicare, a measure that most Democrats opposed as a giveaway to the large drug companies.
Baucus stood with fellow Democrats in 2005 when Bush proposed legislation to partially privatize Social Security, an epic battle that ended in defeat for the president's effort.
He played a central role in the enactment of Obama's watershed health care legislation in 2010, although some inside his party complained that precious momentum was lost while he spent months on bipartisan negotiations that ultimately proved fruitless.
More recently, Baucus has expressed opposition to Democratic proposals to use an overhaul of the tax code as a means of raising additional revenue. He was one of four members of his party to oppose the budget the leadership brought to the floor with a requirement to that effect.
On other issues large and small, Baucus' voting record reflected his rural state.
Most recently, he voted against legislation that Obama backed to expand background checks for gun purchasers.
During the debate on the budget, he was the only Democrat to vote for a proposal to reopen White House tours. Most members of his party viewed the GOP measure as an attempt to embarrass Obama, but it would also have meant more money for clearing snow from the entrances to Yellowstone National Park, a portion of which is in Montana.
For more than a decade, Baucus has sought federal assistance for the residents of Libby, Mont., where asbestos contamination from a vermiculite mine has been linked to deaths and illnesses.
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."
Manoeuvring 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.
Gouras reported from Helena. Associated Press writers Matthew Brown in Billings, Andrew Taylor, Donna Cassata and Alan Fram in Washington and Carson Walker in Phoenix contributed to this report.Suggest a correction