Thousands of residents are being urged to lobby Loudoun County to say no to the Silver Line.

Brent Bennett, Central 7 Station superintendent, crosses the track as work continues on the Washington Metropolitan Area Transit Authority (WMATA) Silver Line project in Tysons Corner, Va., on Thursday, May 10, 2012. (Nikki Kahn/The Washington Post)

“Loudoun cannot afford this bail-out to rail-station developers,’’ according to the script. “If the Loudoun County board opts out, the rail will still be built to Dulles Airport, and commuters will still be within five miles of Metro. Come tell the board of supervisors to opt out and save taxpayers billions of dollars.”

Audrey Jackson, state director of Americans for Prosperity-Virginia , is concerned that the board will pass a “major tax increase” to fund the phase two project.

The board will meet Monday to hear residents’ comments about the Silver Line, but no decision is expected until later in the summer. Americans for Prosperity is planning a rally outside the government center in downtown Leesburg.

The robocalls, which began Tuesday, were being placed to group supporters, tea party activists and likely conservative voters, Jackson said.

Next week, the Metropolitan Washington Airports Authority, which is overseeing the $6 billion construction of the Silver Line, is expected to vote on whether to press ahead with a controversial project labor agreement.

Loudoun supervisors have said the county won’t make its contribution to the project if the labor agreement goes forward.

Virginia Gov. Bob McDonnell (R) also has said the state will withhold its $150 million contribution to Metro’s planned extension to Dulles Airport unless a union-friendly deal for contractors is dropped.

The first phase of the $6 billion Silver Line is under construction from Falls Church to Reston and is expected to be completed in late 2013. Construction on the second part of the project, which will run to Dulles and into Loudoun County, is expected to start early next year.