Missouri residents head to the polls Tuesday for party primaries, and they will have an unusual choice to make. For only the second time in 40 years, the sitting governor of this key battleground state has a formidable challenge from within his own party.

Missouri Gov. Bob Holden (search) predicts he will win a second term, but first the Democratic incumbent must survive two tough elections.

"He's been made to look bad, more vulnerable by the Republicans," said St. Louis University political science professor Ken Warren. "The Republicans have done a really good job of pounding Bob Holden."

So good a pounding that Missouri State Auditor Claire McCaskill (search) has announced she is running against her boss.

"The governor has an awful lot of people that believe he is ineffective in his job," she said.

The polls show the race is close. McCaskill has won newspaper endorsements and raised more than $2 million.

Whoever makes it through the primary will likely have to face Republican Secretary of State Matt Blunt (search), the son of House Majority Whip Roy Blunt. The younger Blunt faces nominal opposition Tuesday, meaning Republicans are just sitting back to watch the Democrats beat each other up.

Click in the box near the top of the story to watch a video by Fox News' Jeff Goldblatt.