An endorsement by Vice President Gore in the hard-fought Democratic presidential race could give a huge boost to Senator Obama or allow Senator Clinton to deliver the coup de grace to her rivals, but several campaign operatives and former aides to Mr. Gore now see the chances of such a nod fading.

Several former advisers said the Nobel Peace Prize awarded to Mr. Gore in October for his work combating global climate change has catapulted him to national and international prominence in a way that could be tarnished by his taking sides in the primary fight.

“In many respects, he has transcended partisan politics,” a former aide, Christopher Lehane, said. “I think he’ll be extremely sensitive about doing anything that could potentially impact his global brand.”

Another former adviser, who asked not to be named, said Mr. Gore had probably waited too long to make an endorsement without causing a fair amount of chaos and upset. “Jumping into the middle of the political process if the middle of the primaries doesn’t make much sense,” the ex-aide said.

Still, Mr. Lehane said all the Democratic candidates would be eager for Mr. Gore’s embrace. “An endorsement I think would be enormously coveted by anyone in the race,” the former aide said, stressing that he had not discussed the issue with Mr. Gore. “The guy’s an iconic figure now. … He would give anyone in the field the imprimatur not only on the environment but on a whole range of progressive issues.”

In Mr. Gore’s circles, Mr. Obama is seen as most likely to get an endorsement. That is in large part because Mr. Gore and Mrs. Clinton often had a difficult relationship in President Clinton’s White House. A new book, “For Love of Politics,” details infighting between the vice president and first lady from 1993 until 2000, when Mr. Gore sought the presidency and Mrs. Clinton sought election to the Senate from New York.

“There was a lot of tension ongoing between Gore and Hillary Clinton sort of going all the way back to the very beginning,” the book’s author, Sally Bedell Smith, said. “That was very clear to the people around Gore and Gore himself. It became a serious issue when she ran for the Senate and diverted resources from his campaign and fund-raisers. By the end, there was a fair amount of bad blood.”

To this day, there is little sign of warmth for Mrs. Clinton or Mr. Clinton among those closest to Mr. Gore. A longtime confidant of Mr. Gore, Martin Peretz, just penned a piece for his New Republic magazine excoriating the former president and his wife. “The Clintons aren’t experts in giving but in taking. This is something of which Barack Obama cannot be accused,” Mr. Peretz wrote.

Talk that Mr. Gore might come to the aid of Mr. Obama flared again last week, when the Illinois senator made a reference to the former vice president while meeting with voters in Portsmouth, N.H. “I talk to Al Gore about every three weeks,” Mr. Obama said, according to NBC News.

Asked for more details, a spokeswoman for Mr. Obama’s campaign, Jennifer Psaki, suggested that the senator is receiving guidance from the Nobel laureate.

“Senator Obama has a great deal of respect for Vice President Gore’s commitment to and leadership on issues related to climate change and he appreciates his advice,” Ms. Psaki said. She did not address questions about an endorsement. Aides to Mrs. Clinton and another Democratic candidate, John Edwards, did not respond to questions about contacts with Mr. Gore.

In August, Mr. Gore told 02138 Magazine that he was likely to throw his weight behind one of the Democratic primary contenders. “Odds are that I will,” he said.

Asked if he felt obliged to endorse Mrs. Clinton, he said, “No. I have friendships with her and with the other candidates and they’re all on equal footing at this point as far as I’m concerned.”

By the following month, the former vice president was squishier about whether he would give someone the nod. “I don’t know if I’ll make an endorsement or not. I just don’t know,” he told CNN on September 26.

A spokeswoman for Mr. Gore, Kalee Kreider, declined to expand on her boss’s public comments.

One potential complicating factor for the former vice president is that an endorsement for Mr. Obama would snub the candidate who arguably has the most aggressive plan to combat global warming, Senator Dodd. Mr. Dodd, who has shown little traction in opinion polls, is the only contender proposing a corporate carbon tax. Mr. Gore backs that idea and pushed the Clinton administration to embrace a “BTU tax” proposal back in 1993.

An endorsement now would also mean snubbing a potential independent candidate for the presidency, Mayor Bloomberg. At a joint appearance in New York in April, Mr. Gore praised as “gutsy” the mayor’s plans to cut greenhouse gas emissions from the city. Asked whether the presidential candidates were giving adequate attention to the issue, Mr. Gore said, “I think Mike is.”

Former aides said Mr. Gore would also have to consider whether he might needlessly tarnish his image by backing a candidate who could quickly fizzle out. In 2004, the former vice president unexpectedly threw his weight behind an insurgent presidential candidate, Howard Dean. Dr. Dean’s campaign soon cratered.