Virginia Gov. Charles S. Robb traveled to his home county of Fairfax last night to boost the congressional candidacy of Herbert E. Harris II--and nudge it gently to the right.

Harris is running in Northern Virginia's 8th District against Republican Rep. Stanford E. Parris, who turned Harris out of office in the Reagan landslide of 1980 after having lost to him six years before.

Robb's appeal to the conservative, moneyed wing of the Democratic Party could be especially helpful to Harris who faces a well-financed campaign by the incumbent as well as a potentially damaging third-party candidacy on the left. Many of the governor's conservative supporters have shown little enthusiasm for the more liberal former congressman.

Robb and Harris have been personally at odds from time to time, since Robb's controlled style contrasts sharply with Harris's flamboyant, from-the-gut rhetoric.

However, at two well-attended fund-raisers in Mount Vernon last night, Robb warmly greeted Harris and his supporters and congratulated Harris for his support of small businesses, lower interest rates and education.

"He hasn't changed any of his basic principles," Robb said. "But I'm pleased he's emphasizing some of the things I've emphasized last year and in Richmond."

Harris joked about the differences in style when he told more than 100 supporters at a $100-a-person cocktail party that the always soberly dressed Robb must be influential to "get Herb Harris into a dark blue suit on a summer night."

Harris said the fund-raisers were a sign of a unified Democratic Party--"It's different from what it used to be," he said--and he atttributed the unity to high interest rates and Reaganomics. Harris also said his two-year "sabbatical" from Congress had "broadened" him and increased his appreciation for the problems of small businesses.

He denied, however, that he has mellowed since the days when he made headlines with staccato attacks against Big Oil. "I don't believe in mellowing," Harris said.

Harris and Robb left the cocktail reception for a lawn party at the estate of Gerald Halpin, a successful office park developer associated with the moderate wing of the Democratic Party. Several hundred people paid $15 each to listen to a band and stand on a bluff overlooking the Potomac River as Robb and his wife, Harris and his wife and several Democratic supervisors from Fairfax and Prince William counties moved through the crowd.

"They have in the past not always seen eye to eye," said Fairfax Supervisor Sandra L. Duckworth. "I was really pleased with Chuck's reaching out and identifying his goals with Herb's goals."

Robb said in an interview that he has planned no further joint appearances with Harris, but will encourage other Northern Virginians to support him. "I know it was an act of faith for some of the folks who worked for me last year," Robb said, an apparent reference to the party's liberal wing that was initially skeptical of his candidacy. "The least I can do is recognize that although there may be differences in philosophy, ideology, style, we're all Democrats."

The 8th District extends from Alexandria through southern Fairfax and Prince William into the northern tip of Stafford County.