The Amalgamated Transit Union Local 689 president issued the threat Friday, after two other labor actions where a significant number of workers showed up to work well after the starts of their shifts.

WASHINGTON — Metro’s largest union is threatening a three-day strike that could grind the D.C. region to a halt. The union provided no timeline on when such a strike might occur, but an emergency strike authorization is taking place Sunday.

A vote to authorize a strike does not immediately trigger one but gives union leaders the authority to call for a work stoppage that Metro said would be illegal. The union has had problems with its elections in the past.

Amalgamated Transit Union Local 689 President Jackie Jeter issued the threat Friday, after two other labor actions where a significant number of workers showed up to work well after the starts of their shifts.

“I will be shocked if there is not an overwhelming endorsement for a strike authorization,” David Stephen, the union’s communications coordinator said Sunday.

On Thursday, Metro estimated about 20 percent of morning buses and thousands of riders were delayed by the larger, second action.

The strike threat came in response to a follow-up memo Friday from Metro General Manager Paul Wiedefeld, detailing discipline against those who came in late Thursday.

“If you make good on your threat to suspend ANYONE for three days for a single miss, ALL OF LOCAL 689 WILL BE TAKING A 3 DAY SUSPENSION (sic),” Jeter said in an email to Metro management Friday afternoon.

Metro’s policies provide for progressive penalties with a first instance usually resulting in something like a warning.

“I want you to know that out of our 12,000-strong team at WMATA, about 500 L689 employees participated yesterday in a job action in violation of their collective bargaining agreement,” Wiedefeld wrote Metro workers.

Under collective bargaining rules and the contract still in effect that expired two years ago, the union is not allowed to conduct labor actions like strikes that disrupt service. In exchange, there is an arbitration and grievance system in effect to handle disputes.

A strike could give union opponents leverage to dissolve many or all of those protections.

“I sincerely hope that no further action is needed. However, we are prepared to pursue all remedies to protect service for our customers, including seeking relief from the courts and progressive discipline up to, and including, termination. To be clear, this is the last thing I want to do, and I am hopeful that no further action is necessary,” Wiedefeld wrote to employees.

Metro and ATU Local 689 expect arbitrators to rule very soon on a new contract. It is expected to be retroactive to July 1, 2016.

Union leaders and other workers have not stated clear demands for Metro to meet that would avoid further labor actions. Jeter frequently uses fiery and sometimes hyperbolic rhetoric both in person and in emails like the one she sent Friday.

The threat of a three-day strike came closest to making a clear demand of the agency: that Metro not suspend anyone who participated in Thursday’s “late out” for three days if they have not had any other disciplinary history.

Jeter also suggested that 500 workers reporting late to work could be understandable in such a large agency.

“Your assumption that the workers who are fed up with you are mindless followers taking union instruction is offensive and tone deaf,” Jeter said.

The union had expressed both internal and external support for the labor action, and acknowledged the “late outs” on both July 4 and July 12.

Metro Chief Labor Relations Officer John Gilman wrote Jeter Friday morning, demanding the union stop supporting such actions.

“Based on the activities of your members yesterday morning, it is reasonable for WMATA to believe that Local 689 members are engaged in a collective job actions,” Gilman wrote.

“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,” Gilman said.

There is no single thing Metro could do to address the union’s concerns, ATU Local 689 spokesman David Stephen said.

The union’s latest issue was with Metro shifting work locations for custodians out of Metro office space and into places like stations, where riders pass through. While Metro did not reduce the number of positions, the union was concerned it would lead to more private contracting.

Stephen claimed Wiedefeld has not negotiated in good faith over general work rules or abided by certain arbitration or court decisions.

It is not clear how or if the union would decide whether to attempt another labor action up to or including a strike, but it could include a vote by union members.

“Stay tuned,” Stephen said. “There are a lot of possibilities. There’s not only one way that we are looking at addressing the dysfunction.”