Union backing will help Gray put “boots on the ground” ahead of April 1 primary. (Jahi Chikwendiu/The Washington Post)

Incumbent Mayor Vincent C. Gray (D) continues to receive more labor support, gaining the backing of two major D.C.-area unions.

On Friday, he stood with regional leaders of the Laborers’ International Union of North America to accept their endorsement. And on Monday, he is set to appear with leaders of the D.C. Building Trades Council, a coalition of 15 construction trade unions.

Their support is rooted in a frequent Gray campaign talking point: the many cranes visible on the city skyline. “Mayor Gray understands that the only way for D.C. to have a vibrant middle class is for business and labor to work together to bring good jobs and economic development to the District,” said Dennis L. Martire, LIUNA’s top regional honcho, in a statement delivering the endorsement.

While LIUNA has more than 40,000 members in the region and another 30,000 in building trades unions, only a small subset of those are D.C. voters. The value of the unions’ endorsement is seen more in their ability to get “boots on the ground” helping the Gray campaign get out the vote as the April 1 Democratic primary approaches.

Gray also has secured endorsements from major unions representing hotel workers, service workers and city employees.

Meanwhile, in the race for the only open D.C. Council seat, key unions representing city employees split their allegiances.

District Council 20 of the American Federation of State, County and Municipal Employees is backing Darrel Thompson, a former aide to Senate Majority Leader Harry Reid (D-Nev.), for the Ward 6 council seat. The unions representing police officers and firefighters opted to endorse rival Charles Allen, the former chief of staff to outgoing council member Tommy Wells.

Still to be decided: The influential endorsements of the Metropolitan Washington Labor Council AFL-CIO, which will be voting Monday night on their preferred D.C. candidates.