Metro's new Silver Line could deliver up to $386 million in tax revenue to Loudoun County if the county's board of supervisors decides to support the project's extension beyond Washington Dulles International Airport, an updated fiscal report shows.

The second phase of the Dulles Rail project, which would add three Metro stations to Loudoun County, would attract 4,837 housing units, 1.4 million square feet of office development, 670,000 square feet of retail space and 294 hotel rooms by 2040, a consultant hired by the county reported.

The promise of economic development in the outer Washington suburb is one factor the county board is weighing as it considers whether to commit at least $264 million to help build the Silver Line, another $16 million annually to maintain the line and millions of additional dollars in interest on money borrowed by the county to build it.

The county would also have to build three parking garages at the stations at a cost of $38 to $52 million each.

The new all-Republican Loudoun board is currently split on whether to back the Silver Line and must decide by July 4 whether it wants to help extend Metro to Loudoun.

Loudoun County resident Daniel Davies, a member of the anti-Dulles Rail group Loudoun Opt Out, said the county government would lose at least $700 million on the project over the next 30 years.

"When you really look at the numbers here, it's just obviously a bad idea for Loudoun County," he said. "It adds up really fast in the wrong direction."

But supporters say the rail project is about more than just tax revenues, saying it would bring economic prosperity and transportation options to the county.

"Rail transit, and Metro specifically, has generated significant economic advantages for those areas that have it," said Tony Howard, president of the Loudoun County Chamber of Commerce. "A suburban -- almost ex-urban -- community like ours really needs a project like Metro in order to remain economically competitive over the next several decades and generations."

But politics could make the dollar signs irrelevant for Loudoun's board, observers said.

County Board Chairman Scott York and other supervisors say the board will refuse to back the rail line if the group overseeing the $6 billion construction project, the Metropolitan Washington Airports Authority, does not scuttle its controversial preference for union labor.

"If the [labor] issue is still going out there, by [July 4], I will not have the votes to go forward," York said.