The effort to extend Metrorail into Loudoun County cleared a major hurdle Wednesday when the regional authority building the new $6 billion Silver Line bowed to the demands of Virginia Gov. Robert F. McDonnell and scrapped a labor-friendly contracting provision.

But even as supporters of the rail extension were breathing a sigh of relief, another showdown loomed, as Loudoun leaders weigh whether the long-sought concession by the Metropolitan Washington Airports Authority is enough to win over Silver Line skeptics on the Loudoun County Board of Supervisors. Without Loudoun’s support — and money — the second part of the rail line could be in jeopardy.

The MWAA voted 11 to 1 to drop an incentive for contractors who use a project labor agreement, or PLA, in building the second phase of the Silver Line.

For months, the board has been reluctant to reverse itself. But it relented after McDonnell (R) said he would withhold $150 million in funding for the line if the MWAA didn’t drop the union-friendly deal.

Still, Loudoun’s supervisors have not given clear indications of whether they will vote to support the second phase of the project and to put in the county’s share of construction costs, which, according to the latest estimate, are expected to total about $270 million. The board is expected to take a vote July 3.

Loudoun Supervisor Matthew F. Letourneau (R-Dulles) said the MWAA’s vote was welcome news — but by no means a guarantee of Loudoun’s support.

“The PLA was a barrier to entry to even have a discussion,” he said. “The way I would characterize it is that the chances now go from zero percent to fifty-fifty.”

At a meeting Wednesday night, Loudoun supervisors consulted with county staff and financial advisers regarding potential financing and debt structure options to fund the county’s commitment to the Silver Line.

Several financing possibilities have been presented to the board, including gas tax revenue, instituting a countywide commercial and industrial tax, or establishing special tax districts that would encompass areas closest to the Silver Line. Some supervisors have said they want to avoid using tax dollars to fund the project, and draw instead from the county’s general fund.

At the meeting, Supervisor Kenneth D. Reid (R-Leesburg) reiterated his concerns about the lack of existing revenue for phase two development, noting that Fairfax County established a special tax district in 2004 to help fund the Silver Line’s phase one construction.

“It’s very hard to say no if there’s funding in place,” he said. “But there isn’t funding in place, and that’s what worries me.”

If Loudoun pulls out, MWAA officials said it would probably delay the start of construction on the second phase by at least 18 months because funding agreements and parking and bus plans would have to be renegotiated.

The MWAA board had been split over whether to drop the PLA from the deal. Tom Davis, a Virginia board member, said that as he and MWAA Chairman Michael Curto talked to other directors, they conveyed this simple message: “There’s no $150 million and no Loudoun if we don’t take it out,” referring to the PLA. “Do you want to be known as the ones who killed the project?”

Bob Brown, a director appointed by the federal government, was the only board member who voted against dropping the PLA. Board member Denny Martire, who is from Virginia and works as vice president and mid-Atlantic regional manager of Laborers’ International Union of North America, said Virginia’s leaders were “holding a gun” to the airports authority to change its previously approved labor deal. But like the majority of the board, Martire voted for the change, in hopes the project would move forward.

Already, the board had relaxed its position, creating incentives, rather than a requirement, for contractors to accept a project labor agreement. But even that drew mounting opposition in Virginia, where this year’s legislative session was marked by deepening ideological divisions.

To critics, a PLA on such a high-profile project violates at least the spirit of the state’s right-to-work law, which prohibits a company and a union from agreeing to a mandate that workers be union members. To supporters, the PLA was no different than those in place on many state and federal projects — including Phase 1 of the Silver Line project.

Project labor agreements typically ensure certain wages and working conditions while barring strikes and providing managers with flexibility in making work assignments.

The MWAA’ retreat on a PLA for Phase 2 was a win for McDonnell, who for months has been adamant that Virginia’s needs have been ignored in the Silver Line project. McDonnell, who has surprised many by governing as a moderate in his two years in the governor’s mansion, initially tried to strike a compromise by allowing some sort of project labor agreement.

But he pulled back after pressure from conservative Republicans. McDonnell said Wednesday that now that the airports authority has dropped the PLA, the state will send the $150 million as promised.

“I’m very happy that they made the vote today. . . . My job is not to make MWAA happy,” McDonnell said. “My job is to look out for the taxpayers of Virginia.” He said the airports authority had made “poor management decisions” from previous administrations that “dramatically” increase rates on the Dulles Toll Road, which is a key source of revenue for the Silver Line project.

The first phase of the rail line, which runs through Tysons Corner to Wiehle Avenue, is under construction and expected to open in August 2013. But the second part of the project, which is expected to run from Reston to Dulles International Airport and Loudoun County, is already behind schedule in soliciting bids.

Political and business leaders in Fairfax County cheered the MWAA’s decision. Several said they were also more optimistic now that Virginia would contribute more money and that Loudoun’s supervisors would be persuaded to continue with the project.

“I am very, very pleased,” Fairfax County Board of Supervisors Chairman Sharon Bulova (D) said Wednesday. “Hopefully, this will be the beginning of a domino effect, where we’re then going to be assured that $150 million is coming from the state for Dulles rail. And then the next is, I hope that Loudoun County will feel they can go forward.”

Mark S. Ingrao, president of the Greater Reston Chamber of Commerce, said the timing couldn’t be better to help tilt Loudoun’s supervisors in favor of the project. The Reston chamber conducted a survey earlier this year that showed the business community there is overwhelmingly supportive of extending the rail into Loudoun County.

“We’re really thrilled about this,” Ingrao said Wednesday. “So now the Loudoun County board is the last domino to fall, and they don’t have any other obstacles to point to. They’ve been saying, ‘Oh, there’s a PLA,’ but that’s gone. So now all they have is their vote.”

Staff writers Caitlin Gibson, Fredrick Kunkle and Anita Kumar contributed to this report.