Sen. Evan Bayh of Indiana always been a tough, perhaps impossible target for the GOP to knock off in 2010. His father was a senator, he’s been governor of the state, he’s won by huge margins in the past, and his approval rating has usually been high. He’s got a ton of money.

Enter former congressman John Hostettler. He’s a non-traditional Republican in a lot of ways; he supported Chuck Baldwin of the Constitution Party in 2008, instead of John McCain. Intriguingly, Hostettler was one of only a handful of House Republicans who did not vote to authorize the Iraq War; Bayh did. If Hostettler wins the primary, Bayh could find himself attacked from the left on the Iraq War.

The spending in Washington and the mood in Indiana appear to have forced Bayh to sound a bit Tea Partyish: “We can’t keep charging to the national credit card. Washington needs a credible plan to pay its bills. Last week, I stood up in the Senate and told my colleagues so.”

Hostettler will face at least three other Republicans in the GOP primary: state senator Marlin Stutzman, Carmel business owner Richard Behney, and Don Bates, Jr., a branch manager for Wachovia Securities.

It’s hard to see a scenario in which Bayh isn’t reelected, but . . . the mood in the country is strongly anti-incumbent and increasingly anti-Democrat. Ordinarily, a high-profile, well-known, and generally well-liked incumbent like Bayh doesn’t have to break a sweat; 2010 might be a year where he has to sweat some.