The Dulles Metro rail project, one of the largest public works projects in the country, is based in Virginia and funded mainly by Virginians, but most of the people being paid to build it are from Maryland.

About half of the 602 construction workers employed by the lead contractor in December for the $2.8 billion first phase of the project were from Maryland, according to data presented to the Metropolitan Washington Airports Authority, which is in charge of the project. Forty-two percent were from Virginia.

Critics say the chief reason for the disparity is that the airports authority prefers union labor. Maryland has more union workers than Virginia, a right-to-work state where employees can't be forced to join unions.

But it is Virginia, along with Fairfax and Loudoun counties and drivers on the Dulles Toll Road who will pay the biggest share of the project's costs.

"Not only do our commuters pay an enormous bill for this, but in addition, 96 percent of our construction workers in Virginia are non-union, which means all those jobs go across the Potomac," Sen. Dick Black, R-Loudoun, told the Virginia Senate Thursday.

The project contractor used a union-friendly labor agreement on the first phase of the rail line, which is expected to be finished next year and may go $150 million over budget. The authority wanted a similar agreement for the second phase. But the authority was forced to back off this week after Virginia lawmakers who objected to the labor agreement threatened to withhold project funding.

Virginia Republicans say the labor agreement not only violates the spirit of the state's right-to-work law but is putting Virginia contractors at a disadvantage against union shops in Maryland.

Authority officials, however, said the labor agreements help keep the massive project on time and on budget and keep workers safe by forcing construction crews to meet high standards.

A spokesman for the project's contractors, Dulles Transit Partners, said the percentage of workers from Maryland, Virginia, the District and elsewhere fluctuates.

"That data is just a snapshot in time," Leslie Pereira said. "Those numbers can fluctuate based on the needs of the project at any given time."

Tara Hamilton, spokeswoman for the Metropolitan Washington Airports Authority, added, "We are not aware of any correlation between the place of residence and the hiring of workers on the Dulles Rail Project."

Just 3 percent of the rail line's workers are from the District. Authority members and District officials expressed dismay that only 18 District residents found work on the project.

"I find it hard to believe that there could not have been a greater effort made to put our residents to work, especially considering the high unemployment rates in many areas of the city," D.C. Councilwoman Mary Cheh said.