A westward view of Dulles Toll Road from the Wiehle Ave overpass where the Silver Line of the Metro is being extended to Dulles Airport. (Jahi Chikwendiu/The Washington Post)

Loudoun County Board of Supervisors Chairman Scott K. York told a group of business leaders Wednesday that if the agency managing the construction of Metrorail’s new Silver Line insists on giving preference to unionized labor, the county would probably vote to walk away from the $6 billion project midway through construction.

And that could jeopardize extending Metro from Reston to Dulles International Airport, Virginia Transportation Secretary Sean T. Connaughton said.

Speaking to Northern Virginia business leaders at a luncheon hosted by the Greater Reston Chamber of Commerce, York (I) and Connaughton urged the Metropolitan Washington Airports Authority to reconsider giving preference to contractors that agree to bargaining collectively on workplace rules.

The practice, known as a project labor agreement, has caused friction between the airports authority and Virginia, a so-called right-to-work state that prohibits compulsory union membership. The Virginia General Assembly is poised to commit $450 million to the project, but not if the airports authority shows favoritism in making sure that union labor will have a piece of the project, Connaughton said.

Initially, the airports authority wanted to impose a mandatory project labor agreement, but later decided to give a 10 percent grading advantage to bids from contractors that agree to bargain with organized labor.

“That’s not fair,” York said. “It should be a level playing field.”

York and Connaughton said that Virginia officials had no objection to voluntary collective bargaining by contractors but that many were strongly opposed to giving a bidding advantage to contractors with project labor agreements. That could prompt Loudoun’s all-Republican board, including seven new members, to drop out of a project whose importance rivals the building of Dulles International Airport, York said.

“If Loudoun opts out, I think it would be a huge mistake,” York said.

Fairfax and Loudoun counties have about three months to decide whether to extend Metro from Reston to Dulles airport and into Loudoun under a new funding formula negotiated last year with the help of U.S. Transportation Secretary Ray LaHood. The agreement reduced the cost of the project’s second phase to $2.7 billion.

In an interview afterward, Connaughton said a decision by Loudoun to opt out of the project would delay it and perhaps unravel it.

“What you have is a very complex mix of contracts, agreements and permits that are all premised on the project going to Loudoun County,” Connaughton said. He added, however, that he was hopeful a solution would be found. “This project has been in the emergency room several times,” he said.

John E. “Jack” Potter, who took over as the airports authority’s president and chief executive in July, also struck a conciliatory tone. “This board has made decisions indicative of the fact they are listening,” Potter told the group.

Fairfax County Chairman Sharon S. Bulova said that she hoped Loudoun would move forward but that her board has shown its willingness to push on to the airport. She also suggested that a nay vote from Loudoun’s board would be serious but not fatal to the project. A delay could increase its cost, and the remaining partners would have to renegotiate financing, but the project would also be smaller and less expensive, she said.

“But, in the end, Fairfax County is committed to extending the Silver Line to Dulles Airport,” she said.