There has been a groundswell of opposition recently to a proposed parkway that would connect Prince William and Loudoun counties, with many residents sharing similar concerns.

Opponents have said that the Bi-County Parkway, a 10-mile state thoroughfare that would connect Interstate 66 in Prince William with Route 50 in Loudoun, would adversely affect nearby Civil War grounds, threaten part of Prince William’s 80,000-acre protected rural area and create a traffic nightmare for residents.

Supporters say the road is needed to connect two of the country’s fastest-growing areas and to spur economic development.

Those arguments were set aside Tuesday by a key federal official and Prince William supervisors who raised yet another issue, one that again links the future of the parkway to that of another long-proposed road: a bypass that would go around the Manassas National Battlefield Park.

Ed W. Clark, superintendent of the Civil War park, has made clear in recent days that he wants a strong indication from the Prince William Board of County Supervisors that it would agree to close Route 234 through the national landmark, and replace it with a bypass, when the Bi-County Parkway is built.

Clark’s support of the parkway project is key. Although the project would be primarily funded and built by Virginia, the National Park Service must approve the road.

County supervisors responded at their regular meeting Tuesday by agreeing to send a letter to Clark and state officials reaffirming their 2005 position: that for Route 234 to be closed, a bypass around the battlefield must be built first.

Both projects are costly, and the move means that the state, and probably the federal government, would have to put up significant funding before the parkway could move forward. The bypass and the parkway are estimated to cost $300 million each, although some think the price could be much more.

Clark said in an interview that he was pleased with Prince William’s action because it makes the construction of the battlefield bypass more likely.

“The state benefits greatly from this, and the locality benefits greatly because you have a smoother flow of traffic,” he said.

Clark’s position sets up a conflict with the Commonwealth Transportation Board, which oversees Virginia’s transportation planning and spending. In February, the CTB passed a resolution saying Virginia could build the Bi-County Parkway without constructing the battlefield bypass. State transportation officials did not respond to a request for comment Tuesday.

Clark has said that he supports the state’s plans for the parkway only because transportation officials have agreed to close Route 234 through the battlefield. No promises for the bypass have been made.

Clark said that because local opposition to the parkway has increased — and because supervisors’ recent actions indicate an uneasiness about the project — he worries that Prince William could throw a wrench into plans by the state and the National Park Service for the Bi-County Parkway by reopening Route 234 after the state shut it down.

“This is not . . . a handshake and ‘I’ll keep my word’ kind of commitment,” Clark said. “We need that firm resolution . . . that the agreement has to be solid and locked down before we move forward.”

Supervisor W.S. Covington III (R-Brentsville) said it’s unclear whether the board’s reaffirmation of its 2005 position would affect the state’s plans.

“They brought back more of the power to the state level [for transportation issues], so I’m not sure what that means,” Covington said.

He said he is not sure whether Prince William would have the power to reopen Route 234 if the state closed it.

“I’m not going to rely on Mr. Clark’s analysis of the state code,” Covington said.

The National Park Service has said that traffic jams and car horns have no place in a pristine national park. In response, federal officials had planned a bypass around the battlefield. The bypass, however, has never been funded.

Residents and Dels. Robert G. Marshall (R-Prince William) and Timothy D. Hugo (R-Fairfax) have complained that closing Route 234 through the battlefield would be a traffic nightmare. The more than 26,000 vehicles that use the road would be forced onto already congested I-66, they say. Also, neighborhoods would be cut off from surrounding communities if the road closes, they say.

Supervisors said they have arranged to meet Clark and Transportation Secretary Sean T. Connaughton at their July 16 public meeting to answer questions.