HOUSTON (Reuters) – Friday’s statement by the United Steelworkers union, USW (NYSE:) Plc regarding proposals to weaken its 56-year old national program for chemical and refinery worker contracts was made by BP (NYSE.) Plc.

According to USW sources, BP proposed in U.S. local negotiations that there be a waiting period of at least 120 days before a contract expires or a strike can begin.

“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 spokesperson for BP declined to comment on the specifics of contract proposals.

“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.

The union at numerous refineries has won a common expiration day for chemical and refinery contracts since 1966. Common dates raise the possibility that workers could strike at multiple sites simultaneously as was the case in the 2015 nationwide strike.

According to a source from the union, “We are standing the line” and described the BP proposal as taking the expiration date for the common contract’s refineries. This allowed the company to “influence it behind the scenes.”

Although it does not alter the official expiration date of the contract, the 120-day non strike period affects the date in practic. It could limit the USW’s ability to collectively take action with an expiration date that is common.

One company has used so-called “labor peace periods” or no-strike agreements. Exxon Mobil Corp (NYSE:), who locked out 650 workers in its Beaumont refinery, Texas, nearly nine months after an initial 75-day strike period.

Sources within the union said that Exxon wasn’t an example worth following.

The USW is negotiating a new national agreement for 30,000 U.S. refinery and chemical plant workers with Marathon Petroleum Corp (NYSE:), the industry’s lead negotiator.

The USW Local Unions also negotiate with the plant managers on site-specific matters.