Former senator Dan Coats may challenge Senator Evan Bayh in Indiana

Coats said in a statement that he has authorized supporters in the Hoosier State to try to gather the required number of signatures from voters by a Feb. 16 deadline.

National Republicans are convinced that Bayh is vulnerable because of his votes in favor of President Obama's health-care plan and economic stimulus package. Coats gives them a very credible challenger, but Bayh is no slouch, with $13 million in the bank at the end of 2009 and a sterling electoral record that includes two terms as governor and two in the Senate.

The party attempted to convince House Republican Conference Chairman Mike Pence to take on Bayh, but he passed -- perhaps with an eye on a 2012 bid for president.

Bayh's biggest potential weakness is the fact that he hasn't run a truly competitive race in decades and may not have the fire in the belly to do so now, given that he weighed retirement earlier in this cycle.