Gray prepares to embrace John Boardman, executive secretary-treasurer for UNITE HERE Local 25, a union of hotel workers. (Mike DeBonis/The Washington Post)

Vincent C. Gray waded through a crowded room in a downtown office building, full of cheering supporters waving signs and chanting “Mayor Gray! Mayor Gray! Mayor Gray!”

It was a moment that, for the first time in his young reelection campaign, seemed right out of Gray’s 2010 run — in the best way possible. His host Wednesday was UNITE HERE Local 25, the hotel workers union, which was a key backer of Gray’s first mayoral run and is now giving him his first major reelection endorsement.

The hotel workers also become the first major labor group to make an endorsement of any kind in the mayoral race. (D.C. Council member Jack Evans won the backing of the regional Machinists last year, but that union has never been considered much of a force in city politics.)

For a mayor who vetoed a labor-backed “living wage” bill for retail workers and gave relatively late support to a major minimum wage hike, Gray won an enthusiastic welcome from Local 25. “Hotel workers in the District of Columbia are better off because Vincent Gray has been our mayor,” Executive Secretary-Treasurer John Boardman told the crowd of well over 100.

The workers’ support means a ready supply of “boots on the ground” to man phone banks and knock on doors as Gray tries to mobilize his base.

The endorsement is, at least in part, a triumph of retail politics. The Gray administration has committed to including union labor agreements in all the economic development deals it negotiates. The new Marriott Marquis hotel opening this spring across from the Washington Convention Center will be a union shop. So will hotels that could possibly be built on city-owned land at the former Walter Reed Army Medical Center site, the old Franklin School, the St. Elizabeths east campus and a city-owned parcel near Mount Vernon Square.

Those projects together stand to add 1,000 to 1,300 members to Local 25’s ranks, which currently stand at 6,500, Boardman said. Aside from the membership boost, he said Gray’s record on worker issues, even taking into account his positions on the wage bills, has been a good one. “He wants a vibrant middle class,” he said.

Boardman was among the union honchos who attended Wednesday morning’s signing of the minimum wage hike, and it remains to be seen whether the other elements of the labor coalition that helped put Gray into office in 2010 will be with him in the run-up to the April primary. Leaders of the largest union representing city employees, AFSCME District Council 20, say they’re behind Gray, but the police, fire and teachers unions which backed him last time are unlikely to be as ardent in their backing this time around, if they back him all.

Gray demurred when asked whether he expects to attract the same level of union support for his reelection bid, instead citing his efforts to re-establish official and unofficial relationships that were broken during predecessor Adrian M. Fenty’s term.

“We think we’ve treated labor well,” Gray said.