A REPUBLICAN senatorial candidate has triggered a firestorm of criticism, including a rebuff from Mitt Romney's campaign, after he suggested that "legitimate rape" rarely causes pregnancy.

Explaining his complete opposition to abortions, even in the instance of rape, Representative Todd Akin said pregnancy from rape was rare because of biological reactions to "legitimate rape".

"First of all, from what I understand from doctors, (pregnancy from rape) is really rare," Akin told KTVI television in an interview widely distributed by Democrats. "If it's a legitimate rape, the female body has ways to try to shut that whole thing down."

Akin then said that if a woman got pregnant after being raped, there should be consequences for the rapist, but not the unborn foetus.

"Let's assume that maybe that didn't work or something. I think there should be some punishment. But the punishment ought to be on the rapist and not attacking the child," he said.

Democrats pounced, with incumbent Missouri Senator Claire McCaskill tweeting that she was "stunned" by the remarks and her re-election campaign using Akin's interview to ask for donations.

Romney, the Republican White House hopeful, and his running mate Paul Ryan immediately sought to distance themselves from Akin's comments on one of the most polarising issues in American politics.

"Governor Romney and Congressman Ryan disagree with Mr Akin's statement, and a Romney-Ryan administration would not oppose abortion in instances of rape," a campaign statement read.

In a statement later released by her office, McCaskill shunned the "offensive" position advanced by her rival.

"It is beyond comprehension that someone can be so ignorant about the emotional and physical trauma brought on by rape," she said.

And her campaign website urged supporters to make a contribution "so we can ensure that Missouri is represented by a Senator who will fight for women's rights - not set them back 50 years".

Akin later said he "misspoke", but stood by his opposition to abortion.

"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 in a statement released by his campaign.