VIRGINIA ATTORNEY General Ken Cuccinelli II wants to kill off the extension of Metrorail to Dulles International Airport and points west, a project he calls a “ripoff” and a “boondoggle.” He is urging Loudoun County voters, who are on the hook for almost 5 percent of the $6 billion project, to elect local government leaders who will withdraw from the deal, helping, he hopes, to cripple it.

Mr. Cuccinelli is a few years late to the party. The decision to go forward with the 23-mile Silver Line, as the extension is known, was made three years ago, when he was a state senator. He objected then, along with other officials who failed to take into account the Silver Line’s potential to juice economic development and reduce traffic, and they lost the argument.

Unfortunately, Mr. Cuccinelli and other Silver Line skeptics have been afforded an opening to revisit the matter — ironically, by the project’s overseer and leading advocate, the Metropolitan Washington Airports Authority. The authority, which runs Dulles and National airports as well as the Dulles Toll Road, has invited attacks such as Mr. Cuccinelli’s by its politically tone-deaf, fiscally irresponsible decision in favor of a hugely expensive, utterly unnecessary underground Metro station at Dulles. An aboveground station would save $330 million and add scarcely three minutes to the trip of airport-bound passengers, who will comprise just 7 percent of Silver Line users.

The first leg of the extension is under construction and scheduled to open for service in 2013; it would extend from the existing station at West Falls Church to Reston. The project’s second leg is the one in dispute; it would run from Reston to the airport and then west into Loudoun.

There is no doubt about the Silver Line’s benefits. When the first leg opens for service, more than 60,000 passengers are expected to use it daily; by 2025, after the full line is complete, it is projected to serve 91,000 daily passengers, many of whom would otherwise be commuting by car.

The experience of the Orange Line in Arlington and the Red Line in Bethesda are testaments to the economic dynamism that follows the opening of suburban Metro stations in affluent areas. The Silver Line, whose four stations in Tysons Corner are under construction, is likely to have a similarly galvanizing effect. If Mr. Cuccinelli is remotely interested in sustaining and expanding Northern Virginia’s commercial vitality and promoting job growth, killing off the Silver Line is hardly the way to do it.

Mr. Cuccinelli’s business is, or should be, rendering legal advice to his clients in state government. Nonetheless, his gratuitous sniping may be a taste of things to come. The furor over the authority’s decision to build an underground station, which is opposed by virtually all the project’s other stakeholders, including the authority’s own staff, must not be allowed to derail the Silver Line. That’s why it’s so critical that Transportation Secretary Ray LaHood, who is mediating the dispute, achieve a resolution that will clear the final hurdles for the project’s completion.