Virginia Gov. Bob McDonnell urged fellow Republicans in Loudoun County to join in the second phase of the $6 billion Silver Line on Tuesday, just days before the county's deadline to decide.

"It's something we have to do. It's one of America's most critical infrastructure projects," McDonnell told a group of developers, engineers and politicians Tuesday night. "This is the gateway to the nation's capital."

Loudoun's nine Republican board members remain split over the cost and potential benefits of extending Metro into the outer suburb. Loudoun would pay $270 million to extend the rail line from Washington Dulles International Airport and $10 million a year in operating costs.

Though the rail project has been in the works for years, most of the Loudoun supervisors are newly elected and re-evaluating the county's participation. McDonnell acknowledged the supervisors' fears.

"It's one of the most expensive infrastructure projects in the country. Two hundred and fifty million dollars a mile -- that's real money," he said. "Obviously, everybody that has a vote in this is making sure they are good stewards of taxpayers' money."

In recent weeks, at least two previously undecided Loudoun supervisors indicated they might support the project, which would give the board enough votes to approve the extension into Loudoun.

"I'm confident the board will vote the right direction Tuesday," Loudoun Chairman Scott York told The Washington Examiner.

Virginia Secretary of Transportation Sean Connaughton joined McDonnell in urging the Loudoun board to vote yes.

"Given that the vote is next week, we would hope that the Loudoun board would vote in favor of the project," he said.

The governor's support comes despite public disputes with the board in charge of constructing the rail line, the Metropolitan Washington Airports Authority. McDonnell ousted a board member earlier this month, accusing Dennis Martire, a union executive, of misusing public funds to attend overseas airport conferences.

The governor was in northern Virginia for the Dulles Rail meeting and for an appearance with presidential candidate Mitt Romney on Wednesday. McDonnell is widely considered a possible pick for Romney's partner on the Republican ticket.