Republican Missouri senate nominee Todd Akin explains his stance on abortion. KTVI

The Republican party leadership has withdrawn financial and political support from a US congressman who is refusing to resign from a Senate race in Missouri after claiming that "legitimate rape" rarely leads to pregnancy.

As pressure mounted from all sides, Todd Akin, chosen last week as Republican candidate in the closely-fought challenge for the Senate in Missouri, apologised for his remarks but refused to step aside.

"I'm not a quitter," he said on a radio show hosted by former Republican presidential candidate Mike Huckabee. "By the grace of God, we're going to win this race."

He added that the people who had elected him knew he was not perfect and that just because someone made a mistake, it "doesn't make them useless". No-one had called him to suggest he drop out, he said.

His defiance came in spite of criticism from senior Republicans, including White House challenger Mitt Romney and senator John Cornyn, the head of the National Republican Senatorial Committee, which provides funding for candidates. Barack Obama also denounced his remarks.

Both the Republican party and prominent political action committees, known as Super Pacs, said they would withdraw their support. Ad campaigns worth millions of dollars were being scrapped.

Akin went on Huckabee's radio show in an attempt to save his position. He said: "I've really made a couple of serious mistakes here which were just wrong, and I need to apologise for those. There is no such thing as legitimate rape. It's an evil act and it's committed by violent predators." He added: "I've even known some women who have been raped and I know that it's a terrible, terrible thing."

In spite of the row over his comment that "legitimate rape" rarely results in pregnancy, he argued his background and ability made him a big asset in unseating the Democratic incumbent Claire McCaskill.

Republicans want him to step aside because the row is threatening the White House race, and reduces the chances of taking the Missouri seat, one the party is banking on to secure a majority in the Senate.

In a statement, Cornyn said: "I recognise this is a difficult time for him, but over the next 24 hours, congressman Akin should carefully consider what is best for him, his family, the Republican party, and the values that he cares about and has fought for throughout his career in public service."

Mitch McConnell, the Republican leader in the Senate, also urged him to rethink his position, describing his comments as inexcusable.

Obama, speaking at the White House at a rare press conference, denounced Akin's comment. "Rape is rape. And the idea that we should be parsing and qualifying and slicing what types of rape we're talking about doesn't make sense to the American people and certainly doesn't make sense to me."

He said that politicians, most of whom are men, should not be making healthcare decisions on behalf of women.

While accepting that Romney had also denounced Akin, he said there was a significant difference between Democrats and Republicans on the issue. "The underlying notion that we should be making decisions on behalf of women for their healthcare decisions, or qualifying 'forcible rape' versus 'non-forcible rape' – those are broader issues … between me and the other party."

Asked if he thought Akin should quit, he said he had been chosen by the Republican party in Missouri and the decision was one for it to decide.

Earlier, Romney used an interview with the National Review Online to denounce Akin in unusually forthright language.

"Congressman Akin's comments on rape are insulting, inexcusable, and, frankly, wrong," Romney said. "Like millions of other Americans, we found them to be offensive."

He added: "I have an entirely different view. What he said is entirely without merit and he should correct it." But he stopped short of calling for Akin to step down.

Republican senator Scott Brown became the first member of the party to call publicly for Akin to resign. "Rep Akin's statement was so far out of bounds that he should resign the nomination for US Senate in Missouri," Brown said in a statement.

Brown's response is not surprising, given he is well to the left of the party, fighting to retain his seat in a tight race in liberal Massachusetts.

Wisconsin senator Ron Johnson also called for Akin to pull out. On Twitter he said: "Todd Akin's statements are reprehensible and inexcusable. He should step aside for the good of the nation."

His intervention was more significant as Ryan is from Wisconsin and the latest poll suggests Romney has taken a narrow lead over Obama in the state.

But even figures such as Karl Rove, George W Bush's strategist, told Fox News that Akin's "got some real explaining to do".

Barack Obama's campaign team portrayed the comments by Akin as further evidence of what it has dubbed the Republican "war on women".

The row began when Akin, in an interview with the St Louis channel KTVI-TV broadcast on Sunday, was asked about his opposition to abortion, even in the case of rape.

"It seems to me, from what I understand from doctors, that's really rare," Akin replied. "If it's a legitimate rape, the female body has ways to try to shut that whole thing down. But let's assume that maybe that didn't work or something: I think there should be some punishment, but the punishment ought to be of the rapist, and not attacking the child."

Akin won the Republican primary only this month with the backing of Tea Party groups. The Missouri race, against Democratic Senator Clare McCaskill, was seen as one of the most winnable for the Republicans in its bid to take control of the Senate.

Akin is only narrowly ahead in the polls and pollsters predicted his remark is almost certain to lose him a lot of votes, especially among women.

But the row could also have implications for the White House campaign as it has focused attention on an area of sensitivity for Romney: the views on abortion of his running mate Paul Ryan.

In a statement released after Akin's remarks, Romney's campaign team 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."

But the statement leaves questions about Ryan unanswered. Until the statement was released Ryan had been opposed to abortion even in the case of rape. Ryan was the co-sponsor of a House bill last year called the No Taxpayer Funding for Abortion Act that included what it referred to as "forcible rape", though the phrase was later dropped after a backlash from women's groups.

Romney has an ambiguous record on abortion, promising while campaigning for governor of Massachusetts not to change existing law but adopting a tougher position when campaigning to become the Republican presidential nominee.

Ryan, by contrast, has been consistently opposed to abortion in almost all circumstances.

Akin's opponent, Claire McCaskill, a close ally of Obama's, said that Akin had not addressed several issues raised by his original comments, including his comment that women's bodies closed down during rape.

She called on Akin to apologise for that and other contentious statements, but rather than join the calls for him to quit she warned the Republican leadership that any attempt to oust him as the candidate would backfire.

McCaskill, seeking re-election, was widely regarded as vulnerable but her opponent's remarks play into her strategy of painting him as an extremist, provided he remains the candidate.

Congresswoman Debbie Wasserman Schultz, head of the Democratic National Committee, sought to broaden the issue beyond Missouri politics, accusing Republicans of wanting to "take women back to the Dark Ages".

She added: "The real issue is a Republican party – led by Mitt Romney and Paul Ryan – whose policies on women and their health are dangerously wrong."