Thousands of hotel workers in a dozen U.S. cities are preparing to go on strike as early as Labor Day weekend as they battle for new contracts with major hotel companies.

Roughy 15,000 workers who are members of the union Unite Here have voted to authorize strikes against Hilton, Hyatt, Marriott and Omni properties. The union is warning travelers to be ready to encounter picket lines in the coming weeks, and possibly over the holiday weekend.

The cities where workers may walk off the job are Baltimore, Maryland; Boston, Massachusetts; Greenwich and New Haven, Connecticut; Honolulu and Kauai, Hawaii; Providence, Rhode Island; Oakland, San Diego, San Francisco and San Jose, California; and Seattle, Washington.

Unite Here, which represents 265,000 workers in the U.S., says this is the first time hotel workers in so many different cities have given the green light to strike simultaneously.

The union is demanding significant raises for housekeepers, food-service workers and other members, along with measures to limit their workloads. Staffing levels are also an issue: Workers have said many hotels cut back on services like daily housekeeping during the pandemic, making their jobs less stable and harder to survive on.

Gwen Mills, the unions president, recently told HuffPost that the industry had largely recovered but was trying to make long-term cuts that could permanently change hotel work.

A lot of the fight is about the workload issues, the cutting of services, and just what the hospitality industry is going to be post-pandemic, Mills said. [Workers] are going to do what it takes to get the economic standards they deserve.

Contracts have already expired at most of the hotels in question, and the remaining ones are set to expire this Friday and Saturday. Strikes could start anytime thereafter.