Todd Akin, a Missouri congressman, ignored calls from party leaders, including Romney, to step down from a crucial election race due to outrage over his claims that rape victims were unlikely to become pregnant and therefore require the right to an abortion.

The growing dispute derailed Romney's attempts to focus on Barack Obama's alleged responsibility for America's economic woes and unemployment crisis. It also threatened to tarnish his moment in the national spotlight next week by drawing attention to party elements that could alienate crucial swing voters.

Romney said Akin should resign, pointing out that senior colleagues in his state party had come to the same conclusion. ''Today, his fellow Missourians urged him to step aside,'' the Republican presidential candidate said. ''I think he should accept their counsel and exit the Senate race.''

But in the face of the intense pressure to quit, Akin only rekindled anger over his comments on tuesday night, by claiming that he had been the victim of "a little bit of an over-reaction", and pledging to fight on.

"I misspoke one word in one sentence on one day," he said in a radio interview. "I haven't done anything that was morally or ethically wrong, as sometimes people in politics do."

As  Romney scrambled to contain the row ahead of his party's gathering in Tampa, Florida, next week, Mike Huckabee, a former Republican presidential candidate, attempted to defend Akin by saying that "extraordinary" people had been conceived during rapes.

"Even from those horrible, horrible tragedies of rape, which are inexcusable and indefensible, life has come," said Huckabee, a former Arkansas governor and favourite of the party Right, who has been given a high-profile speaking slot at the convention.

And with unfortunate timing, officials re-affirmed the official Republican stance that the US constitution should be amended to outlaw all abortions, without exceptions for cases of rape or incest.