The Metropolitan Washington Council of the AFL-CIO decided Monday night to endorse former council member Vincent B. Orange in the at-large race, a significant boost for a campaign that appears to be gaining momentum.

A spokesman for the organization, which represents 180 local unions and activists groups, said the decision was made after union officials interviewed council member Sekou Biddle (D) and former Ward 1 advisory neighborhood commissioner Bryan Weaver, a longtime community activist.

With 150,000 members in the Washington region, the council’s decision will help fuel Orange’s get-out-the-vote efforts in the weeks leading up to the April 26 special election. Orange is competing against Biddle, Weaver and six other candidates in the race to replace D.C. Council Chairman Kwame R. Brown (D) as one of four at-large members of the council.

In January, the D.C. Democratic State Committee voted to temporarily appoint Biddle over Orange as Brown’s successor pending the special election. But Biddle, who has the backing of Brown and Mayor Vincent C. Gray (D), faces trouble on several fronts as he tries to win a full term.

On March 10, Orange reported that he had $191,000 in the bank to spend on his campaign, three times as much as Biddle reported. A Clarus Research Group poll released last week also found Orange, who served on the council from 1999 to 2006, with a double-digit lead over Biddle.

Until now, unions appeared to be splitting their support between Biddle and Orange. Last month, the 45,000-member Service Employees International Union local 32BJ announced that it was backing Biddle. The hotel workers are also supporting Biddle.

But the American Federation of State, County and Municipal Workers threw its weight behind Orange this month. In making its decision, the council sends a signal that the broader labor movement is consolidating around Orange.

Orange, a former executive at Pepco, also appears to have maintained solid ties to the business community.

On Monday, the D.C. Chamber Commerce announced that it will not be endorsing a candidate in the at-large race. But the chamber noted in its news release that it has supported Orange and Republican Patrick Mara, also a candidate, in previous contests.