Workers at one of the Navy’s largest shipbuilders overwhelmingly voted to strike, rejecting Bath Iron Works’ three-year contract offer today and threatening to further delay delivery of ships.

The first strike by Machinists Union Local S6 in two decades comes as production already has fallen six months behind, partly because of the coronavirus pandemic. The private shipyard is one of two that make guided-missile destroyers for the Navy.

The contract was to expire at midnight Sunday for 4,300 workers represented by the union, about two-thirds of the workforce.

The three-year proposal by Bath Iron Works would have given the production workers a 3% raise during each year. But the union’s major issue wasn’t with wages, pensions or benefits.

Instead, the union objected to more than a dozen changes that it considered to be concessions — including hiring of subcontractors. The union also considered a proposal to change preferences for shifts and locations to be an attack on seniority.

Company President Dirk Lesko said the company hired 1,800 workers last year and is hiring another 1,000 this year. But he said subcontractors are still needed to help the shipyard get back on schedule.

He blamed production delays for losing a lucrative contract for Navy frigates to a shipyard in Wisconsin.

Changes governing temporary work assignments were proposed to keep workers busy to avoid layoffs during the inevitable lulls for specific skills during construction, he said.

The last strike by the union, in 2000, lasted 55 days.

The union’s voting took place online and by telephone instead of at a centralized location because of the coronavirus. Voting began Friday and wrapped up at 6 a.m. Hawaii time today.

The shipyard is one of the Navy’s five largest shipbuilders and a major employer in Maine with 6,800 workers.

Workers have become increasingly disgruntled since they made concessions during the last contract amid dire warnings that the shipyard needed to become more competitive to win a lucrative Coast Guard contract. Another shipyard ultimately won that contract.

If anything, the shipbuilding landscape has been even more competitive since. Bath Iron Works, a subsidiary of General Dynamics, competes against Mississippi’s Ingalls Shipbuilding for construction of technologically sophisticated destroyers. But smaller shipyards in Alabama and Wisconsin are also competing for work on smaller warships.

The union contends the shipyard’s problems are caused by management.

The pandemic made existing tensions worse. The union called for a two-week closure of the shipyard, and lawmakers jumped into the fray, urging the shipyard to do more to protect workers.

The federal government classified the shipyard as essential and production, using stringent protocols for disinfecting and distancing, never ceased.

Three workers tested positive for the coronavirus but all of them have returned to work, the shipyard said.