The board of the Metropolitan Washington Airports Authority is expected to take a pivotal vote Wednesday as it decides the fate of a union-friendly labor agreement in the deal to build the second phase of Metro’s new Silver Line.

The vote comes after months of wrangling over the wording of a project labor agreement for the Silver Line’s second leg , which will run from Reston to Dulles Airport and Loudoun County.

The first phase of the 23-mile project is under construction through Tysons Corner and expected to be completed in August 2013.

On Monday, Virginia Gov. Robert F. McDonnell (R) said he would not hold up his promised $150 million for the second phase of the rail line as long as the union-friendly part of the deal is dropped.

In the spring, the General Assembly enacted a law and included language in the state budget prohibiting project labor agreements on the Silver Line project.

Loudoun supervisors have also said they won’t make an estimated $200 million contribution if the labor agreement goes forward.

Project labor agreements, or PLAs, typically ensure certain wages and working conditions while barring strikes and providing managers with flexibility in making work assignments. Such pacts are common in state and federal projects, and a voluntary PLA has been in place for the first phase of the Silver Line construction.

Starting the process to put out bids for the second part of the project is already delayed. Planners had said they wanted to start soliciting bids in early March but have not done so until the PLA deal is worked out.

Follow updates from the meeting beginning at about 8 a.m.

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