Baucus was elected in 1978 and currently serves as chairman of the powerful Senate Finance Committee. The Post sums up his controversial legacy:
At times infuriating his Democratic colleagues, Baucus worked with Republicans to co-write the Bush-era tax cuts and the Medicare prescription drug plan, but he also served as the lead defender against George W. Bush’s 2005 effort to partially privatize Social Security and played a critical role in writing President Obama’s national health-care plan.
From conservative-leaning Montana, Baucus has voted against Democratic initiatives on some social issues, most recently last week’s effort to create an expanded background check system for gun purchases.
Update: The Washington Post's "The Fix" has a comment from a former Baucus staffer, speaking on the condition of anonymity, claiming the Senator's decision was not electoral.
A former Baucus aide who was in touch with the senator’s staff on Tuesday said Baucus opted to step aside for reasons more personal than electoral. Baucus wanted to spend more time with his family, said the former aide, who spoke on the condition of anonymity to provide a candid take.
And the Progressive Change Campaign Committee, which this week launched a $100,000 ad campaign targeting Baucus for his vote against universal background checks on gun purchases, issued an official statement on the Baucus retirement news via email. "Good bye, Senator K Street," wrote PCCC co-founder Stephanie Taylor. "Max Baucus has a history of voting with corporate interests and not the interests of Montana voters—taking millions from Wall Street, insurance companies, and lobbyists. Montana will finally have a chance to have a senator with its best interests at heart, and we hope Brian Schweitzer jumps into the race immediately."
We'll have more on this story as it becomes available.