Missouri Republican Todd Akin, a six-term congressman running against incumbent Democrat Senator Claire McCaskill, was asked in an interview if there were any circumstances in which he would support a woman's decision to have an abortion.
The 65-year-old, who has said he is Missouri's most conservative congressman, told St Louis television station KTVI it was "really rare" for rape victims to become pregnant.
He said: "It seems to me first of all from what I understand from doctors that's really rare.
"If it's a legitimate rape, the female body has ways to try to shut that whole thing down."
He added that he would prefer that punishment for rape be focused on the rapist, and not "attacking the child".
Mr Akin later claimed he "misspoke" during the interview, although his statement did not specify on which points or comments.
It read: "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."
But his comments brought a sharp rebuke from the campaign of presumptive Republican Party presidential candidate Mitt Romney.
Spokeswoman Amanda Henneberg said: "Governor Romney and Congressman Ryan disagree with Mr Akin's statement, and a Romney-Ryan administration would not oppose abortion in instances of rape."
Ms McCaskill, who is seeking a second term, said: "It is beyond comprehension that someone can be so ignorant about the emotional and physical trauma brought on by rape.
"The ideas that Todd Akin has expressed about the serious crime of rape and the impact on its victims are offensive."