Max Baucus Retiring in 2014

Progressive PAC launched ads against him earlier this month

First elected in 1978, Baucus has been the top Democrat on the powerful committee since 2001. The likely Democratic candidate to succeed him would be former governor Brian Schweitzer, sources said.

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.