The labor union Unite Here, with 450,000 active members including U.S. hotel, casino and restaurant workers, has stayed on the offensive this month in its fight to line up expiration dates on hotel contracts in major cities in order to weaken hotel chains. The union’s “Hotel Labor Advisor” e-newsletter offers “tips” for event planners facing labor disputes at their destination hotel, from threatening the hotel owners with future boycotts of the property to avoiding payment of cancellation damages.

The union’s Web site also offers a look-up service showing which hotels have been struck or have locked out workers, which are on a “watch” list, and which are “safe.”

At present, union locals are boycotting 18 hotels in San Francisco and eight in Los Angeles, with other actions scattered among Chicago, Atlanta and Boston and other cities.

This month, the Convention Industry Council issued a statement condemning tactics aimed at forcing sponsors and meeting planners to boycott hotels. Such tactics “can only damage the great economic impact that the meetings, conventions and exhibitions has on a city’s economy,” said Mary E. Power, CIC president and CEO.

The Los Angeles Hotel Employers Council, representing the eight affected hotels, isn’t taking the situation lying down. Its own Web site includes information on “How to Resign from the Union.”