Updated: 12:31 p.m.
(CBS News) In one of the nation's most closely-watched Senate races, Missouri voters will head to the polls Tuesday to determine the state's GOP Senate nominee. Their choice could have significant bearing on Democratic incumbent Claire McCaskill's re-election prospects -- and by extension, the balance of the Senate at large.
McCaskill is widely considered one of the most vulnerable Democrats in the Senate: In a state that has become increasingly conservative since she was elected by a razor-thin margin six years ago, the Democrat has been targeted for her support of President Obama's health care and stimulus bill, as well as a general association with the president, who is not expected to win the state. She also bore the bunt of much criticism when it was revealed that she had in a private plane she co-owns.
According to a recent survey by the St. Louis Post-Dispatch/News 4 poll, McCaskill is currently trailing all three of her competitors in head-to-head match-ups.
Despite McCaskill's difficulties, no clear Republican front-runner has emerged in the race for the GOP nomination. Former state Treasurer Sarah Steelman, and Rep. Todd Akin are attempting to catch businessman John Brunning, who according to a handful of recent polls has a slim edge over his rivals.
The prolonged GOP contest has given McCaskill ample time to raise money and hone her message. It has also given her an opportunity to inject her own influence into the Republican contest.According to Politico, McCaskill been flooding the airwaves with ads aimed at pumping up the conservative credentials of Akin, the candidate she calls in an ad "the most conservative congressman in Missouri." McCaskill has released ads targeting all three candidates, but Democrats widely believe that Akin would be her weakest competitor in the general election. Helping Akin get the nomination could ultimately aid McCaskill in winning the race.
"Akin's no doubt the best [for McCaskill]," said Democratic Missouri strategist Roy Temple. "I think he is the clearest contrast; I think he is the most rigid; I think he presents poorly tonally."
Some polls show Brunner's already-tenuous lead winnowing in recent days, and Akin appears to have gained momentum. Steelman, meanwhile, recently secured the endorsement of Sarah Palin.
"Sarah [Steelman] is an economist who defends our tax dollars -- like a mama grizzly defends her cubs," Palin says in a Steelman ad. "Sarah Steelman earned her reputation as a conservative maverick, blocking pork barrel spending. And Sarah will fight for a constitutional amendment that forces Congress to balance the budget."
According to Temple, it's hard to say which Republican would be the most dangerous for McCaskill's re-election prospects: Brunner, who has no history as an elected official and is casting himself a political outsider with a strong business background, is untested on the national stage. He's also poured about $7 million of his own wealth into the campaign, according to the Associated Press.
Steelman, on the other hand, represents something of a "wildcard" candidate, Temple says. Though currently dogged by fundraising issues, she'd likely benefit from a deluge of funding if nominated, and she's able to appeal to the Republican base without alienating more moderate Republicans.
Republicans say that when it comes to McCaskill's re-election prospects, it doesn't matter who ends up with the nomination.
"There is no incumbent Senator, in either party, in a worse political position than liberal Claire McCaskill and we are confident that whichever Republican candidate is chosen today, they will be the next U.S. Senator from Missouri," said Brian Walsh, communications director of the National Republican Senatorial Committee.