Baucus, 72, has been a fixture in the Senate since 1979 and has been the top Democrat on the Finance panel since 1981.
A Democrat with an independent streak, Baucus supported the Bush tax cuts of 2001 and Obama's signature 2010 health care law. He broke with his party this year to oppose both the Senate Democratic budget blueprint and a hotly fought effort to beef up background checks for gun purchases.
The officials spoke about his retirement decision on condition of anonymity because they were not authorized to publicly disclose the senator's intentions.
Baucus' retirement opens up an opportunity for Republicans to claim a Senate seat in a state where GOP presidential nominee Mitt Romney easily defeated Obama by 12 percentage points last year. But Democrats have proved resilient in Montana, with Sen. Jon Tester winning re-election last year. The election of Steve Bullock last year is the third term in a row in which Democrats have held the governorship.
Former two-term Democratic Gov. Brian Schweitzer would be a top candidate should he choose to run to replace Baucus.
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 have announced they will not seek another term: Jay Rockefeller of West Virginia, Tom Harkin of Iowa, Frank Lautenberg of New Jersey, Carl Levin of Michigan and Tim Johnson of South Dakota.
Among Republicans, Saxby Chambliss of Georgia and Mike Johanns of Nebraska have decided to retire.
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