THERE IS something unreal in watching policymakers dither, and in some cases recoil, at a decision that would create thousands of high-paying jobs and ignite economic growth for years into the future. That is what is happening in Loudoun County, where the nine Republicans on the Board of Supervisors are hung up over whether the county should participate in Metrorail’s Silver Line extension to Dulles International Airport and, if they agree, into Loudoun itself.

Their decision, due by early July, is critical not just to the future of the county but to Dulles rail itself. If they give the thumbs up, it would help clear the way for construction of the project’s second half, probably starting in 2014. If they opt out, it would likely delay the groundbreaking for at least 18 months, and, in the process, sharply increase the costs of an undertaking already estimated at $5.6 billion.

The Silver Line is key to the economic vitality of Northern Virginia and the commonwealth. But the supervisors understandably are concerned mainly with whether the deal makes sense for Loudoun. After all, the county would be on the hook for more than $200 million in construction costs in the project’s $2.8 billion second phase, plus $11 million a year to help finance Metro’s operating costs once the line is complete in 2018 or so. In return, two Metro stations would be built in eastern Loudoun, providing a transit link between the county and Tysons Corner, Arlington, downtown Washington and suburban Maryland.

Opinions are divided on how much the Silver Line’s impact would supercharge Loudoun’s own economy. One economist, Stephen S. Fuller, the director of George Mason University’s Center for Regional Analysis, argues that the benefits would be enormous — $3 billion annually in increased economic activity by 2020, $11 billion annually by 2030, $25 billion annually by 2040. That economic shot in the arm would derive from new businesses and thousands of high-paying jobs that the Silver Line would attract to the county, which in turn would buttress the county’s tax base, alleviating pressure on homeowners paying property taxes.

Others challenge Mr. Fuller’s projections as too optimistic. Maybe. But in other parts of the region Metro has juiced the local economy. There is every reason to think it would do so in Loudoun as well.

Some residents of Loudoun fear the traffic and urbanization that the Silver Line would bring. They like the county the way it is — suburban and prosperous. But growth is coming Loudoun’s way regardless; the Silver Line would only ensure that Loudoun enhances its prosperity as it grows.

The project’s opponents have been encouraged by right-wing groups with scant knowledge of Loudoun and no stake in its future. Their only interest is ideological: they oppose tax increases for any purpose. A group funded by the conservative Koch brothers has sponsored thousands of robocalls in the county, saying the Silver Line would mean a “bailout to rail station developers.”

We suppose that the Koch brothers would also have opposed building the transcontinental railroad, the interstate highway system, the Golden Gate Bridge, or, for that matter, the Metro system and Dulles International airport themselves. Maybe things would be simpler that way. But America would be a poorer, less vibrant place — hidebound, parochial and stuck in old ways.