Mayor Michael Bloomberg is almost certain to face a legal challenge if he tries to alter the city's term-limits law and seek four more years in office.

Several lawyers and government watchdog groups said Wednesday they are mulling legal action to block any changes without the approval of voters, who passed a two-term cap by referendum in 1993.

Bloomberg was expected to announce Thursday that he will ask the City Council to pass a bill giving him and other officeholders the option of running for a third consecutive term.

Even before the specifics of the mayor's plan have been revealed, the idea has already inflamed some critics who are promising a fight.

Public advocacy lawyer Norman Siegel said he has received calls from several people urging him to file a lawsuit, including a political candidate whose campaign plans would be disrupted by a change in term limits.

"The legal question is, can you undo a public referendum by legislative fiat?" Siegel said.

He promised "a hard look" at a legal challenge, a vow repeated Wednesday by other attorneys.

"Lawyers all around the city are going over this with a fine-toothed comb," said Gene Russianoff, a senior attorney for the New York Public Interest Research Group.

Veterans of similar fights, however, say Bloomberg's opponents might not find much solace in the courts.

State and federal judges in New York have a history of rulings that would seem to affirm the City Council's authority to extend or repeal term limits without going back to the voters.

A state appeals court ruled in 1961 that Buffalo's City Council could legally repeal voter-approved term limits without holding a new referendum.

A lower-level appeals court backed New York's City Council when it made minor alterations to the term limits law in 2002 to erase a quirk that would have limited some of its members to no more than six consecutive years in office, rather than eight.

"This isn't a close case. This is open and shut," said Robert Joffe, an attorney who helped represent the City Council during that court fight.

He said courts have repeatedly held that legislative bodies in New York have a right to repeal old laws in favor of new ones.

"A law is a law, and it doesn't matter if it was passed by the City Council or by referendum. They have the same weight," Joffe said.

Raymond Dowd, another attorney involved in the 2002 court fight, also saw few opportunities for a successful challenge.

"It sounds to me like they would have a really, really tough time," he said, though he added that because the stakes were so high, an attempt to get the courts to intervene might be worth the long-shot odds.

One possibility, some lawyers suggested, would be to try to persuade the courts to change their thinking on whether any change to term limits requires a voter referendum.

State law currently requires a referendum in cases where someone has proposed changing the length of an officeholder's term, but the courts have interpreted that provision to apply only to changes in the number of years in a single term, not how many terms someone is allowed to serve.

Siegel said there are other possible grounds for a lawsuit, including the argument that it is too late in the election cycle to make changes without infringing on the rights of candidates who have already begun to campaign.

"I think there will be litigation, one way or another," Siegel said. He urged any member of the public opposed to term limit changes to make his or her voice heard now.

"Any change would be undoing the people's will," he said.