Republican Rep. J. Kenneth Robinson of Virginia's 7th Congressional District has never had to campaign in Manassas before, and that is why he and his Democratic challenger, Lindsay Dorrier, a 39-year-old prosecutor from Charlottesville, are spending a lot of time this year in the outer reaches of the Washington suburbs.

The linking of Manassas, Manassas Park and the western half of Prince William County -- population 55,000 -- with Robinson's Shenandoah Valley-based district is the most significant change in Northern Virginia's political boundaries accomplished by last year's congressional redistricting.

Few expect the new voters in the district to change its rock-ribbed conservative ways, perhaps best exemplified by the lingering influence of its most famous native political family, the Byrds of Winchester. The Prince William precincts voted heavily for Republican Rep. Stanford Parris in 1980, as well as for President Reagan.

But local politicians welcome the change, arguing that, whoever wins Tuesday, the new alignment promises the election of yet another congressman pledged to look after the interests of Washington commuters.

"I think it is very important," said state Sen. Charles Colgan, chairman of Dorrier's campaign in Manassas. "It brings the 7th District closer to Washington and to the people of the metropolitan area. The congressman elected here will have to pay some attention to our problems."

Indeed, Linda Hobgood, Robinson's campaign manager, says the six-term representative has been going out of his way to campaign in the new Prince William districts. "I would say for every event I've booked him elsewhere, I've booked him twice in Manassas," said Hobgood.

Dorrier, a long-shot candidate who has raised only $50,000 so far, has been a dogged campaigner, walking through every town and city in the district, even canoeing up the Rappahannock River into Fredericksburg at the height of a thunderstorm last week. He is pressing 66-year-old Robinson on his "lack of accessibility," challenging him in vain to debate and charging that he has not adequately represented the district.

There is an independent candidate in the race, David Toscano of the Citizens Party, but his campaign appears to have attracted little support.

"A New, Working Relationship," is Dorrier's theme but the handsome University of Virginia law school graduate is careful not to level any personal attacks against the popular Robinson. His campaign literature points out basic differences in their political philosophies: he notes Robinson supports tuition tax credits, school prayer (Dorrier is against) and has opposed the Equal Rights Amendment and various environmental programs that the Democrat would support, including an "indefinite moratorium" on uranium mining in Virginia.

The current economic slump has also been an issue -- not so much in Prince William County, where the jobless rate is less than 5 percent, but in other parts of the district where layoffs by auto-related industries have pushed unemployment up to 15 percent.

On Monday, Robinson, one of the staunchest conservatives on the Virginia delegation, will get a boost from his close friend, retiring Sen. Harry F. Byrd Jr. Byrd will be the star attraction at a lunch of 300 Robinson supporters in their home town of Winchester. It is, the senator's office said, the only campaign appearance in Virginia this fall by the elusive, but still-influential Byrd.