THE COMPLETION of Metro’s Silver Line extension to Dulles International Airport and beyond, a huge project critical to the airport and the region, is starting to unravel. Loudoun County, on whose land the planned Metro station at Dulles sits, has taken a tentative step toward canceling its commitment to the deal. That would jolt the entire undertaking. The Obama administration, which has already pumped nearly $1 billion into the project, is stepping in to mediate.

The tremors stem from the profligacy and obstinacy of the unelected members of the authority that runs the region’s airports, which is in charge of building the Silver Line. In a decision oblivious to the political and economic climate, the Metropolitan Washington Airports Authority board voted 9 to 4 last month for an underground rail station at Dulles. In doing so, it rejected a far more economical aerial station that would save a third of a billion dollars with no meaningful compromise to convenience or design.

The price for the Silver Line’s second leg, running west from Reston to the airport and into Loudoun, now stands at $3.5 billion. Although the airports authority promises to seek savings, the project’s history of spiraling costs suggests the ultimate price will be higher. About three-fourths of the bill will be paid by commuters on the Dulles Toll Road, which the authority also operates.

The authority thinks well-heeled commuters can afford virtually any toll increases. The question is whether they will or should accept indefinite hikes. The answer is no, judging from Loudoun officials, whose constituents are heavy users of the toll road. Members of the county Board of Supervisors, furious that the authority opted for the costly underground station at Dulles, have asked county officials to analyze the implications if the county pulls out of the project.

Loudoun’s withdrawal would be a losing proposition for everyone. The county would lose two Metro stations. The authority would lose the county’s estimated $300 million contribution toward the Silver Line’s construction. Loudoun’s withdrawal would diminish the odds of securing a $1.7 billion federal loan for the project’s second leg, without which tolls will go through the roof.

There are further complications. What would happen to the 5,000 parking spots planned for the two stations west of the airport if those stations aren’t built? Surely, many Loudoun commuters will still want to board Metro; what parking will be there for them? What would be the ripple effect in Fairfax County, also on the hook for hundreds of millions of dollars of the Silver Line bill?

Virtually no one outside the airports authority board supports the underground station. Not state or local officials. Not Rep. Frank Wolf (R), who has championed the Silver Line for years. Not even the airports’ professional staff.

The risk to a vital project in which the federal government has invested heavily has attracted the attention of Transportation Secretary Ray LaHood, who has offered to mediate in a meeting of Silver Line stakeholders next month. Mr. LaHood brokered a deal this year in a bitter standoff between airline executives and politicians involving the multibillion-dollar modernization of Chicago’s O’Hare International Airport. Here’s hoping he can pull that trick off twice.