President Ahmadinejad’s appearance at Columbia University is rekindling a dispute over diplomacy among the leading Democratic presidential contenders, with Senator Obama saying he would meet with the rogue leader during his first year in office and Senator Clinton panning the idea. Mr. Obama said yesterday that he wouldn’t have invited Mr. Ahmadinejad to speak at his alma mater, which happens to be Columbia, but would still meet with him as president.

“It is in the United States’ interest to make certain that we can stabilize the situation there and avoid further military confrontation and curb some of the state-sponsored terrorism that they’ve been involved with,” he said in New York after receiving an endorsement from the city’s Correction Officers’ Benevolent Association. “Meeting with somebody is not tantamount to agreeing with them.”

Mrs. Clinton said she would not invite Mr. Ahmadinejad to a university if she were its president, and distinguished herself from Mr. Obama by reiterating her disapproval for his idea of meeting with the president of Iran.

“We need a much more vigorous and robust and deep engagement, but that does not mean that the president of the United States should take part in such preliminary talks,” she said in Washington yesterday.

The sparring over diplomacy has been one of the few substantive policy differences to separate Mrs. Clinton and Mr. Obama during the campaign.

Other presidential candidates condemned Columbia’s decision to invite the Iranian leader to speak. Mayor Giuliani said in an interview with Sean Hannity that Mr. Ahmadinejad should never have been invited to the university, adding that it “is a damaging thing doing something like this with someone as deranged as Ahmadinejad.”

Fred Thompson said in a statement that Columbia “gave a public forum today to a tyrant to spread his lies and deceit.”

Senator McCain said in a statement that he finds it “astonishing and astounding” that Columbia would welcome the president of a country that is dedicated to a policy of extinction of the state of Israel, and is exporting weapons to Iraq that are “endangering and taking the lives of brave Americans.”

Mitt Romney launched an ad yesterday highlighting his opposition to a visit to Harvard University in 2006 by a former Iranian president, Mohammad Khatami.