The latest Mason-Dixon poll, released Monday, shows Democratic state auditor Claire McCaskill and incumbent Sen. Jim Talent in a dead heat, each with 43 percent of the vote. A late September poll by Zogby International showed Talent leading McCaskill by just 1 percentage point, well within the margin of error.

The razor-thin status of the race is inspiring national activists from both parties to pour money into the Show Me State. In the last two weeks alone, national Republicans have spent $2.3 million backing Talent. The National Republican Senatorial Committee spent $1.3 million this week on a television ad attacking McCaskill. The spot, called "Area Code," argues that McCaskill says one thing about gun control and methamphetamine in the 816 area code–an urban cut of the state–and something else entirely in the 573 area code, a rural part of the state. "She just tells you," the ad says, "what you want to hear."

McCaskill narrowly lost a gubernatorial race in 2004, and back then she accused national Democrats of not spending enough on her in the final weeks of the race. This time, Democrats have spent almost $1 million on her campaign. They also have given about $625,000 to the state's Democratic Party, which will be in charge of building a get-out-the-vote operation that McCaskill says she hopes will be a lasting legacy of her campaign.

Brad Coker, a Mason-Dixon pollster, says the latest poll numbers, which show 13 percent of voters undecided, are "a warning sign" for Talent. "Undecided voters typically go more for challengers than for incumbents," he says. Of Talent's low-40s poll numbers, he adds, "Those are not very impressive numbers for an incumbent."

–Angie C. Marek