San Francisco Mayor Gavin Newsom on Monday vowed to campaign against the interests of the city's hotel owners and join the picket line if management fails to end a lockout of about 4,000 workers by 2 p.m. on Tuesday.

Newsom said the labor fight could hurt the city's tourism industry during the critical holiday season.

He pledged to pressure officials from California and other states to avoid doing business with the hotels if they did not end the lockout.

"I am going to be a strong advocate against their interests because they are being strong advocates against the city's interests," Newsom told a news conference. "We will do everything in our power to make it a national issue."

San Francisco hotels locked out about 4,000 workers after the Unite Here union ended a two-week strike earlier this month. That took to a new level a contract conflict seen as a bellwether for the industry.

The San Francisco hotel group includes properties from brands operated by Fairmont Hotels & Resorts Inc., Four Seasons Hotels Inc., Hyatt, Intercontinental Hotels Group Plc, Hilton Hotels Corp., Omni Hotels and Starwood Hotels & Resorts Worldwide Inc. .

Locals of Unite Here union in Washington, D.C., and Los Angeles are also in contentious contract talks and closely following the fight in San Francisco, since all are engaged in a struggle to change relations with big hotel chains.

At the beginning of October, San Francisco workers struck four properties and were locked out of 10 others that together form a 14-member negotiating group.

Hotels owned or run by most major upscale brands have properties in talks in one of the three cities, where unions want two-year contracts that would end at the same time as contracts covering workers in seven other major markets.

But hotels have adamantly opposed that kind of a deal on grounds it would give unions too much negotiating power in 2006.

Atlantic City casino hotel workers, who are part of the same union, are on strike over a similar issue as they try to line up casino contracts nationwide.