"I think it would be the worst mistake that could be made," said Carter. "That would just accumulate the negative aspects of both candidates." Carter, who formally endorsed the Illinois senator last night, cited opinion polls showing 50% of US voters with a negative view of Clinton.

In terms that might discomfort the Obama campaign, he said: "If you take that 50% who just don't want to vote for Clinton and add it to whatever element there might be who don't think Obama is white enough or old enough or experienced enough or because he's got a middle name that sounds Arab, you could have the worst of both worlds."