A westward facing 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/WASHINGTON POST)

With the clock ticking on whether Northern Virginia’s two richest counties will go along with the next phase of extending Metrorail to Dulles International Airport, all eyes are on Loudoun County’s recently elected, all-Republican Board of Supervisors.

If the vote were held today, elected officials and business leaders in Fairfax and Loudoun counties say they would not be surprised if Loudoun’s board opted out of the second phase of a $2.7 billion project that would extend the new Silver Line from Reston.

Citing uncertainty over future ridership and potential costs, at least two members of Loudoun’s nine-member board have indicated their opposition. Three supervisors lean toward approval. The rest are undecided.

Several said they need to know more about the cost Loudoun will bear for Metro’s operating subsidy.

“All these years, there has never been any serious discussions between Loudoun County and Metro on what our subsidy would be,” said Supervisor Kenneth D. Reid (Leesburg). Reid, who wrote in a newsletter this month that he never liked the project, said county staff estimate that the annual operating cost for Loudoun will be at least $20 million.

But the most serious potential deal breaker hinges on whether unionized labor would be given an advantage in securing a piece of the project, which is being managed by the Metropolitan Washington Airports Authority. In screening bids, the airports authority wants to give a 10 percent bonus to contractors who agree to collectively bargain work rules, under what is known as a project labor agreement.

A voluntary form of such an agreement has been in place for Phase 1. But several Loudoun supervisors said that if the airports authority insists on such agreements for Phase 2, board Chairman Scott K. York (At Large) will probably not be able to muster enough votes to keep the county from walking away. Giving a preference to unionized labor would also jeopardize efforts to commit $450 million from Virginia.

“That message has been delivered in various ways to the airports authority board, but I don’t know if it’s getting through,” said Supervisor Matthew F. Letourneau (Dulles), whose constituents along the corridor generally support the project. “I would hate for that to be the issue that really ends up derailing this — no pun intended — but I think it could be if MWAA doesn’t drop its insistence.”

Yet some fear that opting out could be even more disastrous. Supervisor Ralph M. Buona (Ashburn) said commuters would pay higher tolls and have no Metro.

“That’s almost what I consider the ultimate nightmare scenario,” he said.

The Loudoun board has until July 4 to decide whether to proceed under a new funding formula worked out last year in negotiations with U.S. Transportation Secretary Ray LaHood. With seven new members, the Loudoun board asked for more time. Fairfax and the airports authority had agreed last week to an extra 30 days.

On Thursday, Virginia lawmakers also moved to secure an additional $300 million in bond funding. If the Senate Finance Committee’s deal holds up, Virginia would commit $450 million, including $150 million pledged by Gov. Robert F. McDonnell (R).

Sen. Janet D. Howell (D-Fairfax) said she hoped the extra money would persuade Loudoun to proceed. The Fairfax County Board of Supervisors is almost certain to press on, although there has been a growing chorus of concern about the airports authority’s ability to control costs and navigate Virginia politics.

In a hearing before the Fairfax board on Tuesday, some argued that the entire 23-mile extension is a critical piece in Northern Virginia’s economic engine.

“The Dulles corridor can become essentially a 21st-century main street for the national capital region,” Leo Schefer, president of the Washington Airports Task Force, told the board. “I think you are facing a decision which, in the next 10, or maybe the next 50 years, will be one of the — if not the most — important question affecting Fairfax County.”

The Fairfax County Chamber of Commerce, the Greater Reston Chamber of Commerce, and the Associated Builders and Contractors also support the second phase.

But others urged Fairfax to opt out or risk a financial quagmire. They said population density along the proposed line is insufficient to support heavy rail, and they warned that localities are taking on more of the financial burden as commitments from the federal and state governments have grown shakier.

Even rail supporters worry that higher tolls on the Dulles Toll Road — which will contribute 75 percent of the funding — could drive away business and force traffic onto smaller arteries that are already clogged.

“This is probably the worst financially planned project I’ve seen in 40 years,” said Thomas Cranmer, a representative of the Fairfax County Taxpayers Alliance.

The airports authority has estimated that tolls will rise to $4.50 in 2013-17 and $6.75 by 2018 to pay for the rail line. Cranmer said his numbers suggest tolls would have to jump to $11 one way by 2018. That means Fairfax motorists would pay $5,000 a year, while Loudoun’s would pay $8,250, Cranmer said.

“This is going to be a terrible project for citizens of both counties,” he said.

Metro’s entire Silver Line extension has an estimated cost of $6 billion. Fairfax would bear about 16 percent of the project’s cost; Loudoun would pay nearly 5 percent. Phase 1, which will extend Metro to Wiehle Avenue in Reston, is expected to cost about $2.9 billion. Phase 2, which would run more than 11 miles from Wiehle Avenue to Dulles and then into Loudoun, is projected to start construction in January.