Historic Civil War parkland slotted for a controversial new parkway that would connect the counties of Prince William and Loudoun has made the “endangered” list of one of the oldest non-profit preservation organizations in the country.

Preservation Virginia, founded in 1889, focuses on the preservation of historic sites around the state, including Jamestown and the Cape Henry Lighthouse in Virginia Beach. For the first time, the group included land slated for the proposed Tri-County Parkway, a 10-mile, four-lane thoroughfare that would connect I-66 in Prince William with Route 50 in Loudoun, on its list of “most endangered” sites for 2013.

Part of the parkway, which has drawn increased scrutiny in recent weeks, would run through historic Manassas Battlefield land and rural Prince William.

“The Tri-County Parkway would run directly past the August 28, 1862 position of the right flank of Confederate troops led by Stonewall Jackson and the left flank of the Union General Pope’s troops, taking up to 20-35 acres of land from the national park and historic district,” the group said on its Web site.

“Opponents of the highway...believe that it would negatively impact the national park and historic district and predict that the parkway and connecting roads will open up rural land in Prince William ... and Loudoun County.”

The group joins a chorus of preservation advocacy groups raising concerns about the project, including the National Trust for Historic Preservation, National Parks Conservation Association, Piedmont Environmental Council, Coalition for Smarter Growth, and Southern Environmental Law Center.

The administration of Gov. Robert F. McDonnell (R) and the business community in Prince William and Loudoun believe the road is vital to the success of the fast-growing region. Supporters say the parkway -- which could eventually connect farther east to Interstate 95 -- would create jobs and drive economic development in the area, ease congestion and provide a key connection to Dulles International Airport and between two rapidly growing counties.

Elizabeth Kostelny, the executive director of Preservation Virginia, said that the organization is interested in the project in part because the National Park Service has pushed for assurance that if the parkway is built, Route 29 through the battlefield would be closed at Route 234 and a bypass around the park would be built.

“We’re not opposing it outright,” Kostelny said of the Tri-County Parkway. “We remain concerned about the traffic through the Manassas battlefield [and] having assurances those roads will be closed to commuter traffic.”

The Prince William Board of County Supervisors recently delayed a vote on Prince William’s state transportation priorities due to an outcry about the road. The parkway proposal has long had the support of both Prince William and Loudoun supervisors.

Prince William Board Chairman Corey A. Stewart (R-At Large) said in an interview that the board’s delay does not mean that supervisors plan to pull their support. He also said that despite setback and opposition, he believes the proposed parkway will move forward.

“I think they will be successful,” he said of the state’s plans for the road. “The reason is this ... we have two of the fastest growing counties in the United States that do not have adequate connections to each other.”

Despite opposition in recent weeks -- including from six state area Republican legislators and U.S. Rep. Frank R. Wolf (R-Va.) -- state officials say they plan to press forward and hope to explain their plans for the parkway more clearly and how it would benefit residents.