Republican Rep. Stan Parris is doing something this fall he wouldn't have dared consider in his previous congressional races: campaigning outside his Northern Virginia district.

For Parris, who won his last two races by slightly more than 1,000 votes, the decision to leave the 8th District and campaign elsewhere is statement of striking confidence. And it comes from a candidates whose own aides predicted last year that state Sen. Richard L. Saslaw, his Democratic challenger, would offer a tough and close race.

Parris's plan is to spend some of the remaining campaign days in other parts of Virginia, heading up a get-out-the-vote drive for President Reagan and other GOP congressional candidates.

Some politicans say Parris is so confident of victory that he has turned his sights toward his longtime goal of running for governor and is using this campaign to get known statewide. Others say Parris is merely using the best strategy he can to win reelection -- aligning himself as closely as possible to President Reagan, who is enormously popular in Virginia.

When asked why he is planning to spend time outside his normally volatile district, Parris responded: "I have no higher priority than to get the president of the United States reelected."

Saslaw, 44, who operates several service stations, acknowledged that he is the underdog, but said Parris shouldn't be "cocky" and "overconfident. I plan to win this thing."

"It's not going to be easy, but I've been an underdog time and time again, and I've won eight consecutive races," said Saslaw, who likes to compare himself to Rocky Balboa, the prizefighter portrayed by actor Sylvester Stallone. "And I've won because I've worked hard."

Saslaw, who has represented Fairfax County in the General Assembly since 1976, also acknowledged he has had trouble raising the $500,000 he said earlier this year he would need to wage a credible campaign against Parris. He has raised about $200,000, compared to $700,000 raised by Parris.

The lack of funds has led to a dramatic change in his campaign strategy, forcing him to drop his door-to-door campaigning, a Saslaw trademark, and to spend most of his time on the phone raising money or appearing at various fund-raising events.

In another attempt to jump the campaign into high gear, Saslaw's campaign staff has changed its radio commercials. Earlier ads were sharply critical of Parris' congressional performance. Saslaw's most recent ad does not even mention Parris' name, but instead focuses on Saslaw's major legislative accomplishment during the 1984 legislature -- a bill requiring that women give informed consent before undergoing surgery for breast cancer.

Parris has a $125,000 television and radio advertising campaign planned for late October. He has conducted a low-key campaign, keeping joint appearances with Saslaw to a minimum to avoid giving his opponent additional media exposure. He has scheduled nine debates with Saslaw, in contrast to the 23 he appeared in against his longtime adversary, Democrat Herbert Harris, in 1982.

After their first debate at Temple Beth El, Parris found himself in a hornet's nest. He was asked his definition of a "Christian nation," and answered that a Christian nation was one that did not discriminate. He added that he did not think there should be a national religion.

But because he did not repudiate fundamentalist preachers like the Rev. Jerry Falwell for calling the U.S. a "Christian nation," he was sent an angry letter from four religious leaders, accusing him of making a "highly insensitive and fundamentally misguided statement. . . . "

Parris said a few days later he was visited by Howard Kohr, a representative of the American Jewish Committee, who was concerned about the flap. After their conversation, Kohr wrote a conciliatory letter to Parris, thanking him for his past support on behalf of Israel.

Ever since Harris' defeat in 1982, many had expected Saslaw to be the next Democrat candidate. With his more moderate political image, Saslaw appeared better suited to the district than the liberal Harris, who lost to Parris in 1980 and 1982 by small margins.

Larry Sabato, a political science professor at the University of Virginia, said, however, "There is a Republican tide in Virginia that affects all races. . . . In 1982, we would have handicapped the 8th District race very differently. Northern Virginia is satisfied with the Reagan administration."

The district, which stretches from Alexandria to southern Fairfax, eastern Prince William and northern Stafford counties, is the type of area political scientists say is least susceptible to coattails -- income and education levels are high and the area is rapidly growing, meaning there are fewer longtime ties and loyalties.

But even Democrats acknowledge that Reagan's popularity in Northern Virginia will have some impact on the local races. "The Democrats are not emphasizing the connection with the top of the ticket, that's true," said Fairfax County Democratic Party Chairman Pat Watt. "But how well the top of the ticket does will have some effect on the races."

Parris agreed that Reagan's popularity will help, but he said that coattails are "like a beach crest. The wave is building up, and if we get the surf boards and the surfers lined up just right, you can crest the wave and ride it in. But if you don't, if you're not paying attention, you may miss the wave and not make it."