Bayh's stunning decision – announced Monday afternoon in Indianapolis – came as he geared up for what may have been his most difficult campaign in an otherwise gilded political life.

The son of a senator, Bayh never lost a race over a career in which he was elected as secretary of state at the age of 30 and served as governor and senator for two terms each.

He becomes the fifth Democratic senator to retire, a decision which puts his party, already facing a tough election year, in a difficult position.

Read More: Evan Bayh burns Democrats