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," 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 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 that he would have been equally against an Obama-Clinton pairing if the former first lady had won the nomination, made 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 last 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 just as speculation of a joint Obama-Clinton ticket is building in the US. Lanny Davis, a close Clinton adviser and friend, has launched a petition drive and website - and written directly to Obama - urging him to appoint his defeated rival.

Meanwhile, Bob Johnson, the Clinton backer and founder of Black Entertainment Television, has announced that he hopes 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, as they come 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 likely to be seized on by those Democrats who privately argue that the combination of a black man and a woman on a ticket will represent more change than the US electorate can swallow in one go. This camp believes 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," he said.