THE EXTENSION of Metrorail to Dulles International Airport and beyond, underway in Fairfax County, is one of the biggest public infrastructure projects in America. The venture is vital to this region’s economic development. But at a cost pegged at $6 billion — almost double the estimated price tag in 2004 — it is also generating increasing sticker shock.

That shock is now giving way to political vertigo and pushback, not least because most of the project’s overall cost will fall on commuters who rely on the Dulles Toll Road. Tolls paid by drivers on the road are expected to bear 56 percent of the cost of the extension, which is known as Metro’s Silver Line. Phase II of the project, which will run from Reston through the airport into Loudoun County, relies on tolls for 75 percent of its $3.5 billion cost.

Translation: Toll road users, load up that E-ZPass. Even after adjusting for inflation, a one-way trip, which costs $2 today, is likely to triple or quadruple by the end of this decade, according to preliminary estimates presented to the Metropolitan Washington Airports Authority, which controls the road. That means regular toll road commuters, who now pay about $1,000 a year to drive on the road, may pay more than $4,000 in today’s dollars by the year 2020. And given the Silver Line’s history of ever-spiraling costs, don’t be surprised if that estimate turns out to be low.

Even in the affluent reaches of Fairfax and Loudoun, that may be too much for commuters to swallow. The airports authority, which controls the toll road and is building the Silver Line, says that it is confident that most toll road users can handle the higher bills. In narrow economic terms, they might be right, although a definitive study will not be complete until the end of the year. In political terms, the picture is cloudier.

Part of the soaring tolls — but only part — can be traced to the authority’s recent misguided decision to build an unnecessary and costly tunnel and underground subway station at the airport. An aboveground facility could save as much as $330 million, with little sacrifice in convenience or aesthetics. That decision has ignited a firestorm of criticism from officials in Fairfax and Loudoun, which together will bear 20 percent of the project’s cost, as well as from state officials. Without some resolution of that fight, the project’s financing could collapse.

The airports authority insists it is looking for savings. Its best chance is to secure coveted, low-interest federal loans under the Transportation Infrastructure Finance and Innovation Act. If the feds grant the project the $1.7 billion loan that the authority is seeking, that could hold the increase on the tolls to 250 percent instead of 400 percent. But there is virtually no chance that the project will win a federal loan without a unified push by the airports authority and regional leaders. Given the fight over the underground station at Dulles, the chances for a unified push are nil.

The authority needs to back down before the whole scheme falls apart. It has failed to make a convincing case that the underground station, which would save Dulles-bound rail passengers scarcely three minutes, is worth the staggering price. The authority should not assume that politicians and commuters will accept infinitely higher costs for the Silver Line, no matter what long-term economic benefits it promises.