The Rev. Robert L. Thoburn, the millionaire owner of the Fairfax Christian School, first ran for political office in 1976. He ran again in 1977, and then in 1978, and also in 1979, 1980 and 1981.

From those campaigns, the Republican minister whom former Virginia Lt. Gov. Henry E. Howell once called the "Siamese twin of a caveman" wrested just one prize: a two-year term in the 1978-79 Virginia legislature. This summer as he pursues his seventh annual campaign, Thoburn is far from discouraged.

His conservative allies now control the Fairfax Republican Party, Washington is coming around to his way of thinking and some Republicans say he has a good shot at unseating five-term Del. Robert E. Harris in the Sept. 7 primary.

"I led the way," the round-faced, affable businessman says. "It takes a lot of perseverance."

Thoburn, running in a newly created district that includes Clifton, Centreville and a wedge of Prince William County, is the best known of three conservative Republicans who have mounted challenges to incumbent Fairfax legislators. With as few as 2,000 voters expected to turn out for the post-holiday primary, Fairfax Republicans say Thoburn's proved skill at mobilizing his backers could be decisive.

All three primaries are too close to call, according to Republican Del. Vincent F. Callahan Jr. of McLean, the House minority leader. "It comes down to who can get who out to vote the day after Labor Day, which is going to be extremely tough," he said.

Thoburn in his seventh campaign is shunning the issues that gave him so much publicity--advocating execution for rapists, for instance, or questioning the value of public schools. This year he is not citing the Book of Exodus to support his belief that fathers should be permitted to force marriages between their daughters and the young men who seduce them.

Now Thoburn shuttles from subdivision to subdivision in his black Lincoln Town Car, genially discussing mundane issues like crowded roads and county growth. The real issue, says the 53-year-old minister, is effective representation; Harris is "a phantom at the General Assembly."

Harris, a portly 46-year-old Rockwell International executive who has faced Thoburn in four previous primaries, says he is not fooled.

"He's trying to mask the old Thoburn," Harris says. "He's just trying to put up a lot of blue smoke, and hope the voters don't find him out before election day."

Thoburn, Harris adds darkly, is waging a hidden, negative campaign "in his fundamentalist spheres of influence," sending "political hit letters" to selected conservative families. Harris concedes he never saw such a letter but says several voters told his staff of it. "I'm tracking him real close, closer than the guy realizes," Harris says. "I know how this crowd plays, and I'm not going to get dry-gulched."

Thoburn says he isn't a fundamentalist and never sent a hit letter. "It's a fabrication," Thoburn says. "He's trying to stir up controversy, and then we're called controversial."

Thoburn makes no effort to disguise his conservative philosophy. Government should "protect life and property, enforce contracts, run a court system--and that's it," he says. Other functions, from paying unemployment insurance to delivering the mail, should be handled by the private sector or abandoned.

In addition, Thoburn, an ordained Presbyterian minister, believes the Bible's moral system should underpin state laws. The Bible, he says, provides the basis for everything from prohibiting murder to keeping homosexual teachers out of the classroom to enacting a flat-rate income tax. "Inflation is unscriptural," he says.

Thoburn teaches economics and government at his hilltop school built on 34 wooded acres owned by Thoburn and his wife Rosemary on Popes Head Road south of Fairfax City. The Thoburns began teaching in an Arlington basement 20 years ago and have since schooled the children of conservative leaders like Sen. Strom Thurmond (R.--S.C.) and fund-raiser Richard Viguerie, as well as Heather Harris, the daughter of Thoburn's current opponent.

In the process, the Thoburns have accumulated more than $1 million of property in Fairfax and additional land in West Virginia and Colorado. "We believe in the work ethic," says Thoburn, whose for-profit school charges $2,250 for a year of education emphasizing discipline and fundamentals.

"I'm just a small-business man," Thoburn protested during a house-to-house tour last week after a potential voter said he didn't think much of Christian schools. "We save the county $1.5 million a year, and our kids aren't smoking dope and coming around breaking into your home."

Thoburn's first legislative aide, retired Air Force Gen. Benton K. Partin, is now county Republican chairman. His first campaign manager, Gun Owners of America lobbyist Lawrence D. Pratt, succeeded Thoburn in the House, and two conservative newcomers were elected to the House last year.

This year two more conservatives, retired Army Col. Stephen E. Gordy and Gordon S. Jones, a Capitol Hill lobbyist for what he calls profamily issues, are joining Thoburn in challenging Republican incumbents in adjacent Fairfax House districts. Gordy has taken on attorney John H. Rust Jr. in a district in and north of Fairfax City, while Jones is challenging James H. Dillard II in Burke.

Northern Virginia Democrats have a contest in only one of the area's 21 House districts. James W. Benson and Fred J. Ricci are conpeting to face the winner of the Rust-Gordy contest in the fall. Prince William Democrat Claude (Brad) Bradshaw will face the winner of the Harris-Thoburn race and Democrats have yet to select their candidate in the Dillard-Jones district. The winners will serve a one-year term in the 100 member House.

Dillard, a county school administrator, was first elected in 1971, unseated by a conservative challenge that singled him out as the most vulnerable moderate in 1977 and then reelected two years later.

"Politics has changed in Fairfax County since the right wing came in about five years ago," Dillard says. "It used to be you'd argue the issues before the civic associations and then you'd go out and have a beer with your opponent. Well, you don't have beers with these guys."

Dillard and his allies, who frequently vote with Northern Virginia Democrats for what they see as the interests of the region, say the conservatives cannot work with the Democratic majority in the House and will end up shortchanging Northern Virginia of needed highway and education funds. They say Thoburn's position on the losing end of a 94-to-1 budget vote in 1979 shows he is out of touch.

Thoburn says some votes must be cast on principle, and he condemns Harris as "pragmatic." His two proudest accomplishments in the House, Thoburn says, were proposing a statewide referendum on the Equal Rights Amendment--which never took place--and helping pass legislation to exempt church day-care centers from state regulation.

In any case, Thoburn says, Harris hasn't accomplished much despite his "pragmatism" and nine years' seniority. During the last two years, according to General Assembly officials, only one of Harris' bills was approved: a resolution to designate Jan. 25 as Robert Burns-Scottish Heritage Day.

"Frankly, I don't make a practice of trying to run my score up every year," Harris says. He says he uses his position on the powerful Appropriations Committee to help Northern Virginia in less showy ways.

That doesn't impress Thoburn either. "He Harris conceives of government as a place of contention: who can get the most goodies for the district," Thoburn says. "That sets people up, one group against another . . . . I believe in a harmony of interests."