Two republican candidates who sparked a furore with comments about rape have lost their election bids as the Democrats look set to defend their majority in the US senate.

Missouri Republican US senate candidate Todd Akin, who was criticised in August for comments about "legitimate rape," has been defeated by Democratic incumbent Claire McCaskill.

Until the rape comment, Mr Akin was considered the favourite to beat Ms McCaskill in a state that has trended Republican and voted for Republican presidential candidate Mitt Romney.

But the race reversed course after Mr Akin's comment to a television station that women have natural defences against pregnancy from "legitimate rape".

The comment drew scorn from state and national Republican leaders who called on him to drop out of the race.

Mr Akin apologised for his remarks but refused to withdraw. He regained support of some top Republicans but lost significant financial backing.

University of Missouri political science professor John Petrocik says Mr Akin's opponent was one of the most vulnerable Democrats.

"There is something close to a consensus that he did himself in," he said.

Professor Petrocik says voters moved away from Ms McCaskill after it was revealed she had failed to pay taxes on a private family plane.

People interviewed at St Louis polling stations said Mr Akin's rape comments were important to their vote.

"Seeing [his] attitude toward woman in general, voting for him would be impossible," said Mary Mitchell Bartley, a St Louis historic neighbourhood preservationist who had backed previous Republican candidates.

Mr Akin's loss was another blow to Republicans, who had hoped to make a net gain of four US senate seats to take the majority in the upper house.

In Indiana, another state with a large majority of Republican voters, Democrat Joe Donnelly pulled an upset victory against Republican and Tea Party favourite Richard Mourdock.

Mr Mourdock came under fire after he defended his staunch anti-abortion views by calling pregnancy that results from rape a "gift from God."

Outlining his position, Mr Mourdock said the only exception to a ban on abortion should be for the life of a mother.

"I struggled with it myself for a long time, but I came to realise that life is that gift from God, and I think even when life begins in that horrible situation of rape, that it is something God intended to happen," he said.

ABC/wires