Sen. Claire McCaskill is probably having a pretty good Sunday. Her opponent in the Missouri Senate race, Republican Rep. Todd Akin, has spent most of the day backtracking after saying that victims of "legitimate rape" cannot biologically become pregnant and thus do not need access to legal abortions.

"First of all, from what I understand from doctors [pregnancy after rape] is really rare," Akin told KTVI-TV in defense of his stand that rape victims should not be allowed to access abortions. "If it's a legitimate rape, the female body has ways to try to shut that whole thing down."

Akin said that even if a rape victim does somehow become pregnant, "I think there should be some punishment, but the punishment ought to be on the rapist and not attacking the child."

Akin's comments sparked a big backlash on Twitter, where the hashtag "#legitimaterape" soon became one of the most popular terms on the site. A parody account bearing Akin's head shot mocked the congressman for his comments. McCaskill, meanwhile, also went on the attack. "As a woman and former prosecutor who handled hundreds of rape cases, I'm stunned by Rep. Akin's comments," she wrote.

Akin said in a statement that he "misspoke" in the interview. "In reviewing my off-the-cuff remarks, it's clear that I misspoke in this interview, and it does not reflect the deep empathy I hold for the thousands of women who are raped and abused every year," he said. He later wrote on Twitter that "all of us understand that rape can result in pregnancy & I have great empathy for all victims. I regret misspeaking." (Indeed, a study in the American Journal of Obstetricians and Gynecologists found that rapes result in more than 32,000 pregnancies each year.)

McCaskill's campaign spent $2 million to run ads that boosted Akin as the "true conservative" during the three-way primary race for the Republican nod, which he won by 6 percentage points. McCaskill considered him the weakest potential challenger and wanted him to win the primary, the New York Times reported. The Democratic senator is trailing Akin by about 8 points in the polls, according to TPM's Polltracker.