Tuesday is the deadline for Democrats to get a candidate on the Indiana ballot, but none is expected to file in time. Democratic party leaders will instead choose a candidate by mid-June. Two Indiana House members -- Democrats Brad Ellsworth and Baron P. Hill -- are considered possible candidates to succeed Bayh.
Democrats are considered unlikely to lose control of the Senate, but a growing number of Democratic seats have been put at risk in the wake of the party's bruising loss in Massachusetts to Republican Scott Brown.
Since then, in Delaware, the party's preferred candidate for an open seat -- Vice President Joe Biden's son Beau -- decided not to run. In North Dakota, Democratic Sen. Byron L. Dorgan decided not to seek reelection, leaving the conservative state an almost-certain pick up for Republicans.
Sen. Kirsten Gillibrand (D-N.Y.) is facing a potentially bitter primary fight to win the seat she received by appointment after Hillary Rodham Clinton left the Senate to become secretary of State.