A parkway that would pass near the Manassas Civil War battlefield and connect Prince William and Loudoun counties would only worsen traffic congestion and destroy land that should be preserved, six state Republican legislators said at a news conference Monday on battlefield land.

The so-called “Tri-County Parkway,” a proposed 10-mile thoroughfare that would connect I-66 in Prince William with Route 50 in Loudoun, has been debated for decades. But legislators said that they have become increasingly nervous about the road’s renewed prospects, given new dollars for transportation made possible this year under a plan pushed by Gov. Robert F. McDonnell (R) and passed by the General Assembly.

Del. Timothy Hugo (Fairfax), who represents parts of the affected area, stood with other Republicans outside the Stone House in the Manassas Battlefield Park. Hugo said new transportation dollars should not be spent on “things that are frivolous, that are unnecessary. We need to forget this project.” He also said that officials’ plan to close the stretch of Route 29 that runs through the battlefield would only put increased congestion on I-66.

He said the legislators would fight the road and hope others will join them. “We think this is the next Battle of Manassas,” he said.

Opponents of the project, including area residents, conservationists and smart growth advocates, say the road would open up a rural area to development, and ultimately bring even more congestion. Supporters say the road would create jobs and drive area economic development, ease congestion and provide a key connection between two rapidly growing counties.

Transportation Secretary Sean Connaughton, the former chair of the Prince William Board of County Supervisors, said that the road has been formally adopted into the state’s plan, and has support from the boards in Prince William and Loudoun. It has been that way for years, he said.

State officials hope that the connection will bring more cargo traffic to Dulles Airport. The road through Prince William is part of a longer-range plan to connect Dulles to I-95, and it would allow trucks to reach the airport more easily.

“This transportation facility is desperately needed today and it’s going to be even more critical as we see Loudoun and Prince William grow,” Connaughton said in an interview.

He said the state hopes to move “as quickly as practical” on the road and, while he said he does not know when the entire road would be built, construction could start next year.

Dels. Robert G. Marshall (Prince William), Michael J. Webert (Fauquier) and Sens. Richard H. “Dick” Black (Loudoun), Richard H. Stuart (Westmoreland) and Jill H. Vogel (Fauquier) all joined Hugo in opposing the project at Monday’s news conference. The legislators said that they would lobby the administration and local officials to try and halt the project. That could include a budget amendment in future legislative sessions, they said.

Marshall said that the McDonnell administration was misrepresenting the road’s benefits and that a bridge over Route 28 at Walney and Braddock Roads, among other improvements, would better move traffic toward Dulles Airport than the proposed parkway.

He said that the parkway would only benefit developers who have bought up land along the route. “There’s something else going on here,” he said.

Prince William Board Chairman Corey A. Stewart (R-At Large) said that legislators’ opposition is “a day late and a dollar short.” He said that the two counties are principally connected by Route 15, which he said is “failing.”

“The [parkway] needs to be constructed,” Stewart said.

Some residents who gathered at Monday’s news conference with signs opposing the parkway said they would continue to fight it. Longtime resident Judy Voglsan, 72, handed out literature and said residents would do what they could to make their concerns known.

“We’re going to give it our best shot,” she said.

Legislators said they were equally serious in their opposition. “You don’t often get six Republicans opposing the governor,” Marshall said in an interview.