If New York’s billionaire businessman mayor, Michael Bloomberg, decides to try to change the city charter to allow him to run for a third term as mayor, he will face a clash with another billionaire New York businessman, Ronald Lauder.

The advocacy group backed by Mr. Lauder, New Yorkers for Term Limits, is vowing to defend the term limits law following a report in yesterday’s New York Post that said Mr. Bloomberg is weighing a run for a third term. Current law limits the mayor to two consecutive four-year terms.

“Mr. Lauder remains 100% committed to defending term limits and has every reason to believe the mayor feels the same way,” a spokesman for Mr. Lauder, Nelson Warfield, said yesterday.

New Yorkers for Term Limits has spent about $4 million winning two voter referenda on the issue. In both 1993 and 1996, the public voted in support term limits.

The report in the Post said Mr. Bloomberg may be asking a new Charter Revision Commission to put a referendum on the ballot that would amend term limits so that he could serve a third term. The report said that Mr. Bloomberg, whose current term ends on December 31, 2009, was “50-50” on the possibility.

A spokesman for M r . Bloomberg, Stu Loeser, said, “The Mayor believes in term limits and will be leaving office at the end of his second term,” he said.

Mr. Loeser said that Mr. Bloomberg called for the Charter Revision Commission in his State of the City address in January and that the commission would be given 18 months, a time frame that would not allow Mr. Bloomberg to benefit from any possible change in term limits. The issue of term limits was raised most recently in 2006 when Council Speaker Christine Quinn commissioned a private poll that surveyed 600 New Yorkers about their opinions on term limits. At that time Ms. Quinn said she supported extending the term limit to three terms to two for council members but then later backed away from that position.

Mr. Bloomberg was opposed to the idea then and in 2002 he vetoed a bill that would have allowed eight council members to seek a third term.

At the time Mr. Bloomberg argued that New Yorkers had made their preference clear.

“This would be not in line with Mr. Bloomberg’s positions in the past,” the president of New Yorkers for Term Limits, Allen Roth, said.

The current list of potential mayoral candidates includes Ms. Quinn, Rep. Anthony Weiner, businessmen John Catsimatidis and Richard Parsons, Police Commissioner Raymond Kelly, Comptroller William Thompson Jr., and a state senator, Martin Golden.

Mr. Golden, a strong supporter of Mr. Bloomberg, said Mr. Bloomberg would be a “hypocrite” if he tried to change the term limit law and said the news “keeps him relevant.”

In a statement Mr. Thompson said he “takes the mayor at his word” when he says he will leave office at the end of his second term. Ms. Quinn and Messrs. Wiener and Catsimatidis declined to comment.

The chairman of the New York Conservative Party, Michael Long, said he would oppose the idea of a third Bloomberg term. “We don’t need career politicians in New York City. If the mayor was going to do anything to stay in politics he would look at the governor’s office. If he was to do a charter revision and scrap term limits, it would take away from his accomplishments,” Mr. Long said.

A political science professor at Baruch College, Douglas Muzzio, said he was skeptical that Mr. Bloomberg would change his position on term limits and said the timing of the story was curious, coming on the heels of the defeat of Mayor Bloomberg’s second-term signature issue, congestion pricing.

“Periodically there is news that the mayor is going to do something that boosts his public attention — that he would run president, for governor, for vice president and next it will be the head of the World Bank or the Pope. It is almost like clockwork,” he said.