FOR DECADES regional planners have worked to extend Metrorail to Dulles International Airport, a critical transit link that would juice Northern Virginia’s economy and bring first-world convenience to the Washington region’s preeminent international hub. Now, incredible as it may seem, the Loudoun County Board of Supervisors, seven of whose nine members are newly elected and mostly novices at regional issues, may be poised to delay, and possibly scuttle, the project halfway to completion.

Metro’s $5.6 billion Silver Line, one of the biggest and most important infrastructure projects in the country, is on track for an on-schedule finish next year for its first phase, which extends west through Tysons Corner to Reston. The question is whether Loudoun will remain on board, together with Fairfax County and the authority that runs Dulles, to pay part of the rail line’s second phase, extending to the airport and into Loudoun itself. It ought to.

The all-Republican board has until early July to decide whether the county is in or out. If the board votes in favor, Loudoun would get two Metro stations west of the airport and a huge lift to its economic prospects. If it votes nay, that will be enough to postpone — possibly for a year or two — the project’s 11-mile second phase.

In the worst-case scenario, postponement could kill the deal. At the very least, it would, as Virginia Transportation Secretary Sean Connaughton has said, put it back to square one, forcing fresh legal and financial arrangements and possibly a new environmental study for what would be a reconfigured project.

Even if the pieces were put back together, the region would lose valuable time — and probably miss out on the exceptionally low construction costs and interest rates available now.

The ostensible cause for Loudoun’s wavering is cost. The county would be on the hook for $275 million for the Silver Line’s construction, or 4.8 percent of the overall price tag, plus some $16 million annually in subsidies for Metro once the line is operating.

When Metro officials appeared before the board in Loudoun the other day, they got a grilling from supervisors — by turns distrustful, aggressive and confused — who seemed to think that the Silver Line’s costs were understated and the benefits uncertain. In fact, the costs are modest measured against Metro’s potential for supercharging the county’s economic growth, which is why virtually every major business group in Loudoun supports it.

According to Stephen S. Fuller, director of George Mason University’s Center for Regional Analysis, extending the Silver Line into Loudoun would make the difference between the county continuing mainly as a bedroom community that exports workers into neighboring jurisdictions and transforming part of it into a employment magnet for highly educated and well-paid professionals.

In fact, said Mr. Fuller, if Loudoun turns its back on Metro, the county would sacrifice more than $55 billion in potential economic activity, and some 40,000 jobs, by 2040, and its rate of growth would be 10 percent slower. Should the effort to connect Loudoun by Metro to the employment centers nearby fail, “those jobs would go to Reston or Tysons Corner” or be lost to the Washington metropolitan area, Mr. Fuller told us.

The real reason for the Loudoun board’s hesitation has more to do with politics than economics — specifically, the signals sent by Republican leaders in Richmond. Rather than trying to get the Silver Line built, Gov. Robert F. McDonnell and leading GOP lawmakers have thrown spikes in the project’s path. Mr. McDonnell has refused to make an unqualified commitment to support the Silver Line financially, even as he’s funneled hundreds of millions of dollars to vastly less important downstate road projects.

The result is that Virginia so far has contributed scarcely 5 percent of the Silver Line’s overall projected cost — much less than the commitments made by the federal government or Fairfax County. The state acts as if rail to Dulles were being built on some distant planet, rather than in its own most economically dynamic region. In fact, Virginia itself stands to gain tremendously by the project’s completion.

Mr. McDonnell and his aides like to complain that the Washington Metropolitan Airports Authority, which oversees Dulles and Reagan National airports and is in charge of building the Silver Line, has bungled things. The alleged mismanagement is among the pretexts cited by the state for denying a request by Northern Virginia lawmakers, who sought $300 million in additional state funds for the Silver Line. In fact, the project’s first phase has been fairly well managed, as indicated by the expected on-time completion late next year.

Now the McDonnell administration, which gave preliminary approval last summer to a separate $150 million subsidy, is withholding even that money in a fight over a labor agreement struck by the airports authority that the governor regards as pro-union. While the governor said this month that he supported the Silver Line, his statement, which lacked any financial commitment, was an empty declaration.

Mr. McDonnell’s insidious foot-dragging sends the wrong message to Loudoun at a critical juncture of its decision process. If Loudoun pulls out, sacrificing its two Metro stations in the bargain, the Silver Line’s financing would have to be restructured. That could be done — Fairfax and the airports authority are both highly motivated partners — but it would take time. The project’s price tag would almost certainly rise.

Loudoun will be doing no favors for itself or for the region by pulling out of the Silver Line. It would be, in fact, a spectacular folly — penny-wise and pound-foolish policymaking at its worst. If Mr. McDonnell is remotely interested in economic development in the state’s most dynamic region, he should step in to find a solution — and stop throwing bombs from the sidelines.