THE PRICE TAG to compete Metro’s nearly half-finished Silver Line extension to Dulles International Airport and west into Loudoun County is about $2.7 billion. Of that amount, Loudoun, which would get two Metro stations, is on the hook for just 10 percent. But a feverish debate among newly elected local lawmakers divided over Metro’s costs and benefits has imperiled the county’s participation. Without it, decades of planning aimed at connecting Dulles to the regional transit system may collapse.

Mindful of the stakes, Gov. Robert F. McDonnell (R), who supports the project, has been publicly and privately lobbying his fellow Republicans in Loudoun, who monopolize the county Board of Supervisors. His personal intervention is a hopeful sign, given the sky-high stakes and the expected closeness of the board’s vote on whether to stay the course, which is set for Tuesday.

Extending Metro to Dulles would be a lifeline for the airport, a major stimulus for the regional economy and, in the long run, a boon for Loudoun’s tax base. Yet bizarre as it may seem, just one or two wavering Loudoun supervisors could make the difference between the timely completion of one of the nation’s biggest infrastructure projects and what would be a long and potentially fatal postponement.

If Loudoun pulls out, it would force a rewrite of the Silver Line’s legal, financial and environmental blueprints. For instance, a major Metro rail yard, planned for the line’s terminus in the county, would have to be relocated, probably in western Fairfax County. Additional parking would have to be found for the planned Metro station at the airport. Traffic projections on the Dulles Toll Road would have to be recalculated. All that could take 18 months or longer, according to state and local officials — and by then borrowing costs to finance construction would likely be a good deal higher.

Mr. McDonnell is a longtime backer of Dulles rail, but until recently he hadn’t thrown his full weight, and the prestige of his office, behind it. With some justification, he’s squabbled with the Metropolitan Washington Airports Authority, which controls Dulles and is overseeing the Silver Line’s construction. That’s had the effect of deepening skepticism about Dulles rail among Loudoun’s nine supervisors, all but two of whom took office last fall.

Publicly, Mr. McDonnell’s pitch has been for Loudoun to get behind what he called “one of America’s most critical infrastructure projects.” But all politics is local, and what the fence-sitters in Loudoun need to know is that Metro, despite its hefty upfront cost, will be a good investment for the county.

It is. Metro has been a catalyst for often dramatic economic growth throughout the region, attracting businesses, investment and a highly educated workforce that wants a transit option. Over time, the addition of those enterprises and affluent professionals has beefed up the commercial tax base, easing the pressure on middle-class homeowners.

Passions in Loudoun are running high. One county board member called Metro “evil”; another fired her close aide for supporting the project. Give credit to Mr. McDonnell and his team for seeking to douse those flames and provide a calm, reasoned rationale for finishing the Silver Line.