Barack Obama should not pick Hillary Clinton as his vice-presidential nominee, former president Jimmy Carter has told the Guardian.
"I think it would be the worst mistake that could be made," Carter said, adding: "That would just accumulate the negative aspects of both candidates."
The former president, who formally endorsed the Illinois senator late on Tuesday, cited opinion polls showing 50% of US voters with a negative view of Senator Clinton.
In terms that might discomfort the Obama camp, 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."
Carter, who insisted he would have been equally against an Obama-Clinton pairing if she, not he, had won the nomination, makes the remarks in an interview with the Guardian's Weekend magazine, to be published on Saturday. The interview was conducted before the final round of voting on Tuesday night confirmed Obama as the party's presumptive nominee.
The intervention of the former president - regarded as the senior elder of the Democratic party by some, and as a walking reminder of electoral failure by others - comes as speculation of a joint Obama-Clinton ticket is building. Late last night a close Clinton adviser and friend, Lanny Davis, acting on his own initiative he said, launched a petition and website - and wrote directly to Obama - urging him to appoint his defeated rival.
Meanwhile, Bob Johnson, a close Clinton backer and founder of Black Entertainment Television, said that he hoped to persuade the Congressional Black Caucus - the umbrella group for African-American members of Congress - to lobby for an Obama-Clinton partnership.
Carter's remarks could slow that momentum, coming from the only living Democrat to have won more than 50% of the popular vote in a presidential election, even though the former president, who left office in 1981, insisted he was "on the outside" and no longer had any role in internal Democratic affairs.
His comments are also likely to be seized on by those Democrats who believe Obama needs to pick an experienced, white and probably southern man to "balance" the ticket.
The former president said: "What he needs more than a southerner is a person who can compensate for his obvious potential defects, his youthfulness and his lack of long experience in military and international affairs."
For that reason, Carter says, he favours Sam Nunn, the former chairman of the Senate armed services committee, who hails from his own state of Georgia. "That would be my preference, but there are other senior Democrats who would have similar credentials to Sam Nunn."