Research on the prevalence of rape-related pregnancies is spotty. One estimate published in 1996 said about 5 percent of rapes result in pregnancy, or about 32,000 pregnancies among adult women each year.
McCaskill was ready to move on, saying Akin should not be forced out of the race.
"What's startling to me is that (Republican) party bigwigs are coming down on him and saying that he needs to kick sand in the face of all the primary voters," McCaskill said Monday at a campaign event in suburban St. Louis.
"I want Missourians to make a choice in this election based on policy, not backroom politics."
The McCaskill campaign seemed to favor a matchup against Akin. McCaskill ran statewide TV ads during the primaries painting Akin as too conservative even for Missouri. She also ran ads against his GOP rivals.
The Akin ads served two purposes for McCaskill: boosting Akin among the more conservative Republican primary voters to help get him nominated and raising questions about him among moderates and liberals.
Akin won the state's Republican Senate primary just two weeks ago by a comfortable margin over millionaire businessman John Brunner and former state Treasurer Sarah Steelman. Many considered him a favorite to beat McCaskill in November.
Experts say the rape comments were a game-changer.
"He may in fact have mortally wounded himself," said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University in Carbondale. "This is a statement that is so crude and so offensive to more than half the electorate that there's a real danger here that he has dealt himself out of this race."
University of Missouri political scientist Peverill Squire said Akin's comments could particularly hurt him among suburban voters, where Republicans have done well in recent elections and "where McCaskill really does need to pick up some votes to stay in office. This certainly gives her an opening."
Ushering Akin from the race is complicated by the fact that he has never been a candidate beholden to the party establishment. Since being elected to Congress in 2000, Akin has relied on a grassroots network of supporters. His Senate campaign is being run by his son.
Missouri election law allows candidates to withdraw 11 weeks before Election Day. That means the deadline for the Nov. 6 election would be 5 p.m. Tuesday. Otherwise, a court order would be needed to remove a candidate's name from the ballot.
If Akin were to leave, state law gives the Republican state committee two weeks to name a replacement. The candidate would be required to file within 28 days of Akin's exit.
If Akin gets out, attention turns to Brunner and Steelman, but other possibilities include Missouri Auditor Tom Schweich, whom Republicans unsuccessfully tried to draft into the race earlier this year; former Sen. Jim Talent; and two members of Missouri's House delegation, Blaine Luetkemeyer and Jo Ann Emerson.
Talent, who lost his seat to McCaskill in 2006, said Monday he had been asked to run but replied: "I'm not running for the Senate."
"I'm totally ruling it out," Talent said in Tampa, Fla.
Akin, a former state lawmaker who was first elected to the House in 2000, has a long-established base among evangelical Christians. He has been an outspoken abortion opponent, and his campaign website proudly points out that he is listed among Planned Parenthood's "Toxic Ten" legislators.
Associated Press writers Henry Jackson in Washington; Jim Suhr in St. Louis; Chris Blank in Jefferson City, Mo.; Lindsey Tanner in Chicago; and Tamara Lush in Tampa, Fla., contributed to this report.
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