The airports authority in charge of the $6 billion Dulles Rail project reversed course Wednesday and dropped a union-friendly labor agreement, caving to political pressure from Republican Gov. Bob McDonnell, of Virginia, and the all-Republican Loudoun County Board of Supervisors.

After months of resisting state threats to cut off funding, the Metropolitan Washington Airports Authority dropped its plans to give contractors who favored union labor bidding advantages on the second phase of the rail project, which will extend Metro's Silver Line from Reston into Loudoun County.

McDonnell and state lawmakers threatened to withhold $150 million in funding if the airports authority didn't drop its union preference, which Virginia leaders worried violated the state's right-to-work law. Loudoun County had threatened to pull out of the project altogether if the board didn't abandon the labor agreement.

"This is a really significant decision," Airports Board Chairman Michael Curto said. "Hopefully it will provide the impetus for Loudoun County to vote and opt in to the project. I think we've secured the $150 million from the commonwealth."

Federal, state and local officials welcomed the move -- the second time Virginia officials have pressured the airports authority to reverse a decision. The authority earlier dropped plans to build an underground Metro station at Washington Dulles International Airport when state and local officials complained that it cost about $300 million more than an above-ground station.

"This is a step forward with honoring Virginia law and practice as well as the wishes of the Dulles Rail funding partners," Virginia Secretary of Transportation Sean Connaughton said.

One airports authority member, Bob Brown, voted against dropping the labor agreement, saying the authority had no guarantee that Virginia would pay the promised $150 million.

"This is a history of a partner who moves the goal posts on us repeatedly," Brown said of Virginia. "We need a written commitment from this governor to know if this money is going to be there or not."

Board member and vice president of the Laborers' International Union of North America, Dennis Martire, voted to drop the labor preference because of financial concerns, but he expressed frustration with the political pressure behind the decision, saying the airports authority's backtracking was "offensive" to hardworking contractors.

With Virginia's $150 million contribution, authority members said they would be able to limit increases to tolls on the Dulles Toll Road, a major source of funding for the rail project.