BATH, Maine — More than 4,000 workers went on strike against one of the U.S. Navy’s largest shipbuilders Monday after rejecting a three-year contract. It was the first strike by production workers at Bath Iron Works in 20 years.

Pickets formed at midnight when the old contract expired in a dispute that focused on subcontracting, work rules. and seniority over wages and benefits.

Bath Iron Works already had fallen six months behind on ship construction, partly due to the coronavirus pandemic, and the strike threatened to further delay production of destroyers for the Navy, the company said.

The three-year contract proposal would have given production workers a 3 percent raise each year. But the shipbuilders’ union objected to more than a dozen changes that it considered to be concessions — including hiring subcontractors.

Machinists Union Local S6, which represents 4,300 workers, voted 87 percent in favor of a strike. The tally was announced on Sunday.

“This should send a crystal clear message to BIW management: Respect your workers, go back to the bargaining table and negotiate a fair contract,” Maine AFL-CIO President Cynthia Phinney said in a statement.

Frustration had been building among workers since the last contract in which the Machinists union accepted concessions that were deemed necessary to win a U.S. Coast Guard contract — and save shipbuilding jobs.

Bath Iron Works lost that contract to another shipyard in 2016. It also lost a competition for Navy frigates in late April.

Shipbuilders contend the problems have been caused by mismanagement. The company contends the shipyard must be more efficient and get back on schedule to successfully compete for work.

Competition is becoming fierce. 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 issue at Bath Iron Works is how much the shipyard can absorb and remain competitive, said Loren Thompson, an analyst at the Lexington Institute.

“Management is afraid that they’re going to price themselves out of the market,” he said.

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

Bath Iron Works contends changes in the contract are aimed at making more efficient use of workers and streamlining the hiring of subcontractors.

The shipyard hired 1,800 workers last year and expects to hire 1,000 workers this year, but subcontractors are still needed for the shipyard to get caught up on the construction schedule, company president Dirk Lesko has said.

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 since returned to work, the shipyard said.