The race has been close almost from the start, with one January poll showing Talent trailing McCaskill 44 percent to 47 percent. Talent could be paying for his confusing stance on stem cell research: After getting guff from McCaskill, he decided in February to take his name off a list of cosponsors for a Senate ban on forms of the research.

That brought only anger from conservatives. Republicans have increasingly tried to pelt McCaskill with allegations that her stance on immigration is unclear. Both candidates have stated publicly that they are against the Senate's guest-worker proposal. Talent has served in the U.S. Senate for the past four years, after winning a special election in 2002. That year, Talent defeated Jean Carnahan, who had been appointed to the U.S. Senate seat in 2000 after her husband, former Gov. Mel Carnahan, was killed in a plane crash shortly before the Senate election that he won posthumously because it was too late to remove his name from the ballot.