Baucus, leading supporter of health reforms, to retire

"Just days after calling Obamacare a 'train wreck,' its architect Max Baucus waved the white flag rather than face voters," said Rob Collins, executive director of the National Republican Senatorial Committee. "Obamacare has gone from being an 'abstract' discussion to a real life pain for workers and families, which has Democratic candidates like Bruce Braley, Mark Pryor, Mark Begich and Kay Hagan backpedaling. … The 2014 electoral map is in free-fall for Democrats, who were already facing a daunting challenge."

Possible Republican candidates for the seat are former Gov. Marc Racicot; Denny Rehberg, the former congressman who lost a bitter race last year to Tester; Rick Hill, another former congressman who lost to Bullock; and Steve Daines, the current Montana congressman.

The only Republicans who have declared their intention to run is state Sen. Champ Edmunds of Missoula and former state Sen. and gubernatorial candidate Corey Stapleton.

Democrats in the Senate will be defending 21 seats next year to Republicans' 14, with several Democrats running for re-election in GOP-leaning states that Romney won handily. Among the Democrats facing tough challenges next year are Sens. Mark Begich of Alaska, Mary Landrieu of Louisiana and Mark Pryor of Arkansas.

Democrats also have more retirements than the GOP. Five Democrats in addition to Baucus have announced they will not seek another term: Jay Rockefeller of West Virginia, Frank Lautenberg of New Jersey, Tim Johnson of South Dakota, Harkin and Levin.

Among Republicans, Saxby Chambliss of Georgia and Mike Johanns of Nebraska have decided to retire.

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."

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Despite his standing as a top Democrat in Capitol Hill, Baucus sometimes bucked the party line in recognition of Montana being a fundamentally conservative state with voters who want someone willing to base votes on more than party lines.

"I don't focus on labels," he has said. "For me, Montana comes first and partisan labels are a distant second."

He was an architect of the President George W. Bush's prescription drug plan in 2003, one of the few Democrats to back a GOP-led effort to provide prescription drug coverage under Medicare. The law is now widely popular with Republicans and Democrats.

Baucus is from a wealthy Helena ranching family. He practiced law in Montana in the early 1970s until he was elected to the state House in 1973. He first won election to the U.S. House as part of the huge 1974 Watergate class and easily moved up to the Senate in 1978. He has had only one close race since, when he defeated then Lt. Gov. Denny Rehberg with less than 50 percent of the vote in 1996.

Baucus became an advocate for the residents of the Montana town of Libby after news reports in 1999 linked asbestos contamination from a vermiculite mine there to deaths and illnesses. He helped deliver money to those who fell sick and became a vocal critic of both the W.R. Grace Co., and the Environmental Protection Agency for not doing enough to clean up the town.

He also worked to protect the land bordering Glacier National Park by advocating energy companies to retire their leases in the North Fork watershed of Montana's Flathead River.

Baucus voted in favor of invading Iraq, but said later that his vote was a mistake based on faulty intelligence delivered to Congress. After his nephew was killed while deployed in Iraq, Baucus said in later years that the troops should come home as soon as possible.

Baucus ran afoul of his constituents during President Bill Clinton's administration when he supported a handgun-control law and a ban on the sale of some assault-style weapons. Gun ownership is widespread in Montana, and Baucus later supported allowing those laws to expire in 2004.

Baucus came under criticism in February 2009 when he recommended Melodee Hanes for Montana's U.S. attorney post when he was dating her. Hanes withdrew her name from consideration in March and was hired in June as a top official in the Justice Department.

She and Baucus married in June 2011 at the historic Montana ranch north of Helena run by his family.

Baucus attended Stanford University, where he earned a bachelor's degree in 1964 and a law degree in 1967. He worked as an attorney with the Civil Aeronautics Board from 1967 to 1968, and with the Securities and Exchange Commission from 1968 to 1971. He practiced law in Montana from 1971 to 1974.

He and his ex-wife, Ann Geracimos, have one son, Zeno.

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Gouras reported from Helena. Associated Press writers Andrew Taylor and Alan Fram in Washington, and Carson Walker in Phoenix contributed to this report.