San Francisco Mayor Gavin Newsom threatened Monday to join hotel workers on their picket lines today unless hotel owners end a four-week lockout and allow employees to return to their jobs for a 90-day cooling-off period.

Newsom gave the owners of 14 hotels until 2 p.m. today to respond to his request. If they refuse to go along with the cooling-off period, the mayor said he will seek to have the city stop doing business with the hotels and call for a public boycott.

"If the lockout does not end, the hotels know where I am going to position myself -- as an exceedingly strong advocate for the people out there on the lines, in the rain, through the holidays, who are the pawns in this," Newsom said. "I'm a free market, pro-business Democrat, and I'm trying to be fair here, but my city's being tarnished, and these workers are being hurt."

Mayoral spokesman Peter Ragone said the mayor could join pickets as early as this afternoon.

The union representing 4,000 locked-out hotel workers -- cooks, room cleaners, bartenders, bellmen, servers and others -- said they are willing to return to work for a 90-day cooling-off period. Representatives of the 14 hotels involved in the labor dispute said they would give the mayor an answer by his deadline.

"We understand the mayor feels strong about this,'' said Barbara French, a spokeswoman for the hotels. "We have his request under advisement.''

On Sunday, Newsom proposed the cooling-off period in a letter to Mike Casey, president of Local 2 of the hotel workers union, Unite Here, and Mark Huntley, president of a group of 14 San Francisco hotels that is negotiating labor contracts, the Multi-Employer Group.

The two are locked in a tense labor dispute that led to a two-week strike against four of the hotels that began Sept. 29 and to the eventual lockout of the employees of all 14 hotels -- more than 4,000 in all. Contract negotiations have been fruitless.

"The hotels now have gotten their two weeks in after the two-week strike,'' the mayor said. "Fair is fair," he said. As far as I'm concerned, you're even. Now let's all grow up and get back to work."

In his letter, Newsom said the continued dispute "causes significant disruption to the citizens and visitors of our city, and it threatens to interfere with San Francisco's economic recovery.''

The four hotels that the union originally struck were the Argent, the InterContinental Mark Hopkins, the Hilton and the Crowne Plaza Union Square. The workers were locked out of those four plus the Fairmont, Four Seasons, Grand Hyatt, Holiday Inn Civic Center, Holiday Inn Express & Suites Fisherman's Wharf, Holiday Inn at Fisherman's Wharf, the Palace Hotel, Hyatt Regency, Omni and Westin St. Francis.

There are three conventions this week in San Francisco with a total of 36,000 people attending. But another major gathering -- more than 5,000 delegates of the American Anthropological Association scheduled to meet Nov. 17-21 at the San Francisco Hilton -- might be moved to Atlanta. Organizers of the convention said Monday they want to learn the response to Newsom's request. The Hilton is one of the four hotels where workers struck on Sept. 29 and one of the 14 where the lockout continues.

Casey wrote to Newsom on Monday saying the union would agree to send members back to work from Wednesday through Jan. 25 while negotiations continue. Casey said the return to work would be unconditional and the union would negotiate on all outstanding issues.

Spokeswoman French said Monday the 14 local hotel managers met to discuss the proposal and are discussing it with their own companies.

"The hotels believe the solution is an agreement'' reached at the negotiating table, she said. "They appreciate and respect the mayor's continued involvement.''

Ultimately, Newsom's authority in the matter is limited to his power of persuasion and the prestige of his office, but San Francisco's losses could be considerable in a prolonged labor dispute. The cancellation of major conventions -- and the subsequent loss of tourist dollars -- is still possible.

The American Anthropological Association made reservations for its 2004 annual meeting at the Hilton eight years ago, said Elizabeth Brumfiel, the association president and professor of anthropology at Northwestern University. On Friday, she and the group's executive board sent an e-mail to members saying that, because of the labor dispute, the board had voted to move the meeting from San Francisco to the Atlanta Hilton, Dec. 15-19.

On learning Monday morning of Newsom's overture, the group suspended those plans pending the response from both the union and the hotels, said Brumfiel. Monday, she e-mailed members telling not to cancel or make new reservations until it's decided where to hold the meeting.

However, both sides' agreeing to a cooling-off period today would not automatically translate to a decision to keep the meeting in San Francisco. It may be impossible to change back from the plan to shift to Atlanta, as members began making changes to their itinerary at week's end, Brumfiel said, adding that a final decision about the venue would come today.

The group sides ideologically with the union, and Brumfiel has written that "anthropologists cannot, in all good conscience, meet in facilities whose owners are using the lockout of low-wage workers as a bargaining tactic.''

The group had a legal vulnerability, too, as breaking the Hilton contract would expose the members to potential damages of $1.2 million -- the fee for renting the hotel facilities. The association and Hilton worked out a tentative compromise, said Brumfiel -- go to Atlanta, a nonunion hotel, this year but return to San Francisco in 2006.

However, Debbie Larkin, a spokeswoman for the hotel, said, "Our understanding is that (the anthropologists' meeting) is still going to be here.''

By calculations of the San Francisco Convention & Visitors Bureau, the 5,000-plus anthropologists would probably spend $3,093,750 in the city.

In 2003, 14 million visitors spent $6.3 billion at local businesses, and conventioneers spent close to 60 percent of the total, the rest coming from leisure and business travelers, said Mark Theis, the bureau's convention manager.

John Marks, who heads the bureau, said he thinks both the union and the hotels will have to find an incentive to participate in a cooling-off period. "There has to be some blue sky, some potential win for both sides,'' he said. "You don't want to play nice for three months. That's no big victory.''

Notwithstanding the dispute, there are 6,000 people attending the Mortgage Bankers Association meeting this week, 12,000 at the Cellular Telecommunications and Internet Association meeting, and 18,000 at the Audio Engineering Society meeting beginning Thursday, said Marks.

Chronicle staff writer Wyatt Buchanan contributed to this report.