FORESTVILLE, MD — A showdown looms in Washington now that Metro’s largest union has overwhelmingly authorized a potential transit system strike, just as thousands of tourists arrive in the nation’s capital for this week’s Major League Baseball All-Star Game.

Striking is forbidden under Metro’s bargaining agreement with the union, but even a brief interruption could disrupt a system that carries about a million people a day.

Union leaders said they would wait on Monday’s expected response from Metro’s management after Sunday’s vote authorizing a strike, which comes as frustration mounts after years without a new contract.

The Amalgamated Transit Union represents around 8,000 of Metro’s 12,500 active workers, who have been working without a new contract since July 2016. Local 689 President Jackie L. Jeter says 94% of voters approved the potential strike.

“We will decide the when and where and how,” Jeter said at a news conference Sunday night. “We have to call a meeting of the executive board after this vote, and then we’ll decide on what we’re going to do.”

Because Metro workers are forbidden from striking under the system’s governing compact, a judge or arbitrator could order an end to any strike and penalize those who do not comply.

“We understand the ramifications of what we’re asking our members, we understand what a strike would mean,” said Jeter.

Even a brief work stoppage could cause commuting chaos, particularly as thousands of visitors try to get around Monday and Tuesday nights for events surrounding the All-Star game.

Carroll Thomas, the union’s first vice-president, put it this way: “If we don’t move, this region doesn’t move.”

The strike authorization vote followed co-ordinated “late-out” demonstrations on July 4 and Thursday, in which some workers showed up late for their scheduled shifts, delaying services.

Both sides traded barbs after these actions, which were meant to send a message to Metro management about stalled contract negotiations, job cuts, privatization, duty reassignments and other issues. Metro then threatened to discipline anyone who showed up late again, and Jeter said suspending a single worker for missing a shift would prompt the entire union workforce to take a 3-day suspension.

According to The Washington Post, Metro Chief Labor Relations Officer John M. Gilman reminded Jeter in an email that the union is prohibited from organizing collective service disruptions.

“We demand that Local 689 cease and desist from any further illegal action and that you immediately instruct your members to arrive on time for work and to comply with all standard operating procedures,” his email said.

Since Metro General Manager Paul J. Wiedefeld took over the transit system in November 2015, union members have held regular demonstrations at Metro board meetings, voicing their opposition to cuts to jobs and open positions, fare increases, service cuts and a shift toward private contractors over union workers. The agency faces chronic safety and reliability issues, driving down ridership. The resulting revenue losses triggered fare increases and cutbacks in service.

Forty years have passed since the last Metro workers’ strike, a weeklong “wildcat strike”, a strike without union leadership authorization, in 1978 that disrupted commutes across the region.