The Metropolitan Washington Airports Authority is in charge of the project, and it plans to require all contractors on the project to pay union-level wages.
Local Republicans, led by Rep. Frank Wolf, say that the use of union labor will drive up the cost of the $5 billion project even more. They also argue it would shut out Virginia's contracting firms, the vast majority of which are non-union.
Union activists disagree with that argument. Dozens of them stormed a news conference on the topic at Wolf's office in Herndon Monday.
Loudoun County Supervisor Scott York dismissed the demonstration as a political stunt, saying the real issue is keeping the cost of the project down.
The total cost of the Silver line has been under scrutiny throughout the life of the project, but especially in recent weeks after MWAA opted to build the Dulles Airport Metro station in the more expensive underground location.
"Let me just say one thing to my friends who have come from D.C.: 'Hey Hey, Ho Ho, I want my taxes low!'" York said.
The project to extend Metro out to Dulles Airport and beyond is one of the largest and most expensive public works projects in the nation.
Virginia's attorney general Ken Cuccinelli will face former Democratic National Committee chairman Terry McAuliffe in November to become Virginia's 72nd governor.