President Clinton is ruling out the possibility that he could serve as the Democratic vice presidential nominee in 2008, conceivably as a running mate to his wife, Senator Clinton.

“I just don’t believe it’s consistent with the spirit of the Constitution for someone who’s been president twice to be elected vice president,” Mr. Clinton said last night on CBS’s “Late Show with David Letterman.”

“I don’t think it’s right and I wouldn’t want to do that. …That’s just not in the cards,” the former president added, according a transcript released by the network. A former professor of constitutional law, Mr. Clinton described as “contorted” the contention of some scholars that the Constitution could allow such a gambit.

Mr. Clinton also expressed surprise that polls show Mayor Giuliani as the front-runner in the Republican field, despite warnings from many analysts that his liberal positions on social issues would doom his candidacy with the Republican primary electorate. “He’s proved to be more durable so far than I kind of thought he would be,” Mr. Clinton told Mr. Letterman.

Mr. Clinton also said he admires a Republican contender whose campaign has stumbled recently, Senator McCain of Arizona. “I don’t agree with him about Iraq, but I think he’s independent and does what he thinks is right, and that’s about all you can ask from anybody,” he said. Mr. Clinton, who is on the interview circuit to promote a new book on volunteering and public service, said his wife’s campaign gave him blunt instructions for his appearances: “Don’t make news.”