WASHINGTON — A union representing nearly 600 United Launch Alliance employees is urging its members to reject a proposed contract in a vote this weekend, a move that could set up a strike.

The negotiating committee for the Machinists Union said it unanimously rejected the proposed three-year contract presented by ULA as its best and final offer April 26. The current contract for union members expires May 6.

The union, in its response to that offer, took issue with a number of its provisions, including increased health care costs and contributions to retirement plans. “By voting for this contract, you are letting ULA take away many of your benefits and future money, as well as that of future generations of Machinists,” the union stated in its response.

“Machinists Union members made ULA into the industry leader it is today. We refuse to be disrespected,” said Machinists Union International President Robert Martinez Jr. in an April 27 statement. “The full force of 600,000 Machinists Union members across North America stand with our members at ULA and we demand a fair contract.”

ULA counters that its offer is “well above market and unlike anything offered by our competitors, providing our skilled workforce with increases in nearly every element of the contract,” according to the company’s website. It notes that the contract includes general wage increases of 1.5 to 2 percent per year over the course of the contract, and a $6,000 ratification bonus per employee if members approve the contract before May 7.

The contract covers nearly 600 union members at ULA’s main manufacturing facility in Decatur, Alabama, as well as its launch sites at Cape Canaveral, Florida, and Vandenberg Air Force Base, California.

If members reject the contract, the next step would be a vote to authorize a strike, union spokesperson Deirdre Kaniewski said May 1. She declined to state what vote margins would be needed to either approve or reject the contract or authorize a strike.

That vote would come immediately after ULA’s next scheduled launch, of NASA’s Mars InSight mission on an Atlas 5 from Vandenberg early May 5. There are no Atlas or Delta launches currently scheduled after that mission until late July.

ULA spokesperson Jessica Rye said May 2 that ULA has plans should union members go on strike. “If a strike were to occur, our operations would continue, and we would implement our contingency plan while focusing on meeting our commitments to our customers,” she said.

ULA and the union last negotiated a contract in May 2015. In that vote, 53 percent of union members at the company, at the time numbering 840, approved the contract despite a similar lack of endorsement of the contract by the union’s negotiating committee.

“There are good things and bad things in this offer,” the union said in a statement regarding the 2015 vote. “The contract offer fell short of our expectation and what we wanted to bring back to our members. Therefore, your negotiating committee decided unanimously that we cannot recommend this contract. The final decision, as always, rests with you, our membership.”