HOUSTON, Jan 21 (Reuters) - The United Steelworkers union (USW) on Friday said proposals by energy giant BP Plc would undermine its 56-year-old national program for refinery and chemical plant worker contracts. People in the USW familiar with the matter said BP has put forward proposals in local negotiations at its U.S. refineries to require waiting periods of up to 120 days between the expiration of a contract and the possible start of a strike. “BP"s position at local tables attacks the National Oil Bargaining Program,” the union said in a message to members and seen by Reuters. “Pipeline & refinery locals stand together to fight back!” A BP spokesperson declined to discuss the details of proposals in contract negotiations. “BP is negotiating in good faith with the United Steelworkers union to improve the competitiveness of our business and create a sustainable future for all,” said company spokesperson Cameron Nazminia. A union spokesperson was not immediately available to discuss the USW’s message. Since 1966 the union has at many refineries won a common expiration date for refinery and chemical plant contracts. The common date raises the possibility workers will strike at multiple plants simultaneously, as happened in 2015, during the last nationwide strike. "We"re standing the line," said a union source, who described the BP proposals as taking refineries off the common contract expiration date, allowing the company to "influence the contract behind the scenes." While not changing the formal expiration date, the 120-day no strike period changes the date in practice, potentially reducing the power of collective action available to the USW with a common expiration date. One company that has used no-strike or so-called labor peace periods is Exxon Mobil Corp (XOM.N), which locked out 650 workers at its Beaumont, Texas, refinery nearly nine months ago, following a 75-day no-strike period. "We don"t believe Exxon is the best example to follow," a union source said. The USW is negotiating a new national agreement for 30,000 U.S. refinery and chemical plant workers with Marathon Petroleum Corp (MPC.N), the industry’s lead negotiator. At the same time, USW local unions are negotiating site-specific issues with plant managers.