The most discussed candidate is former Sen. Dan Coats, who Democrats have focused their efforts on tarnishing before he even gets a formal operation off the ground.
Coats was first elected to the House in 1980, and appointed to the Senate in 1989 after Dan Quayle's election to the vice presidency. Coats was then elected in his own right in 1990 and 1992, and then retired in 1998, with Bayh picking up the seat.
Former Rep. John Hostettler, state Sen. Marlin Stutzman, financial adviser Don Bates Jr. and Tea Party activist Richard Behney are already in the race. Behney does not have the needed signatures yet, a source told us.
Conservatives such as RedState's Erick Erickson like Stutzman.
(Hostettler was defeated in 2006 by Rep. Brad Ellsworth, who is weighing whether he'll ask to be considered as the Democratic nominee for Bayh's seat.)
Indiana Republican Party Chairman Murray Clark said Bayh has served "with distinction" and said the fall is looking brighter for the GOP.
"[W]e have always viewed this Senate race as one that would be competitive. Today's announcement cements that view and we really like our chances in the general election," Clark said.
The Democrats will choose their candidate at a State Central Committee meeting June 30, thanks to the late decision from Bayh. That means they will have plenty of time to field the best contender against the candidate Republicans nominate.