Missouri legislator Todd Akin has apologised for his televised comments that women's bodies are able to prevent pregnancies if they are victims of "a legitimate rape", but he refused to heed calls to abandon his bid for the Senate.
Appearing on former presidential candidate Mike Huckabee's radio show on Monday, Akin said rape is "never legitimate".
"It's an evil act. It's committed by violent predators," Akin said. "I used the wrong words the wrong way."
Calls for Akin's exit from the race grew on Monday, with at least two Republican senators - Scott Brown of Massachusetts and Ron Johnson of Wisconsin - saying he should resign the party's nomination.
The six-term congressman in the House of Representatives is a Republican nominee for the US senate, opposing Democratic incumbent Claire McCaskill in the November election.
The election is one of this year's most closely watched races as it represents one of the Republicans' best chances of defeating a Democratic incumbent as they try to gain control of the Senate.
McCaskill, who is seeking a second term, is considered vulnerable because of her strong ties to President Barack Obama, and the fact that Missouri is considered an increasingly conservative state.
'Not a quitter'
Pledging to continue the race against McCaskill, Akin said: "The good people of Missouri nominated me, and I'm not a quitter.
"And my belief is we're going to take this thing forward and by the grace of God, we're going to win this race."
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During the primary campaign, Akin ran TV ads in which Huckabee praised him as "a courageous conservative" and "a Bible-based Christian" who "supports traditional marriage" and "defends the unborn".
Asked in an interview Sunday on KTVI-TV if he would support abortions for women who have been raped, Akin 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."
Later on Sunday, Akin released a statement saying that he "misspoke" during the interview, though the statement did not say specifically which points were in error.
"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," Akin's statement said.
Akin also said he believes "deeply in the protection of all life" and does "not believe that harming another innocent victim is the right course of action".
'Rape is rape'
Moments after Akin's apology, Obama said Akin's comments underscore why politicians - most of whom are men - should not make health decisions on behalf of women.
"Rape is rape," Obama said. He said the idea of distinguishing among types of rape "doesn't make sense to the American people and certainly doesn't make sense to me".
Akin's comments also brought a swift rebuke from the campaign of presumptive Republican presidential nominee Mitt Romney and his running mate, Paul Ryan of Wisconsin.
Romney and Ryan "disagree with Mr Akin's statement, and a Romney-Ryan administration would not oppose abortion in instances of rape," Amanda Henneberg, a spokesperson for the Romney campaign, said.
Romney went further in an interview with National Review Online, calling Akin's comments "insulting, inexcusable and frankly wrong".
"Like millions of other Americans, we found them to be offensive," Romney said.
In an emailed statement on Sunday, McCaskill said it was "beyond comprehension that someone can be so ignorant about the emotional and physical trauma brought on by rape".
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.
Behind the scenes, Republican officials were looking for intermediaries trusted by Akin to try to coax him from the race.
Missouri election law allows candidates to withdraw 11 weeks before Election Day.
If Akin were to leave, state law holds that the Republican state committee has two weeks to name a replacement. The candidate would be required to file within 28 days of Akin's exit.
Akin has a long-established base among evangelical Christians and was endorsed in the primary by more than 100 pastors.