Before he decided to take another plunge into Northern Virginia politics, Vernon H. Miles Sr. wavered.

Two years ago he was trounced by the incumbent Republican, state Del. Robert E. Harris, in one of the most lopsided contests for the General Assembly that year. Did he really want to undergo a rematch in the 40th District?

Miles' doubts lingered, apparently, until the final day he could file as a candidate, June 11. Then, motivated principally by what he said was a distaste for his opponent and a conviction that Harris was doing a poor job in the House of Delegates, Miles made the decision to run again.

"If I don't achieve anything at all," said Miles, a 47-year-old Democrat who is an administrator at the National Academy of Sciences, "I will at least have gotten Bob Harris out into the open where people can see him and evaluate him."

Responded Harris: "The gentleman just has no idea how the General Assembly operates, nor does he have a general idea of state issues and how they relate to the legislature."

Leaders of both the Republican and Democratic parties in Fairfax County say that Miles has little chance of ousting Harris, 49, who has withstood a dozen primary and election challenges since he was elected to the House of Delegates in 1974. Miles himself acknowledges that his chances are at best slim in one of the strongest Republican areas in the state.

So far, the race has attracted little attention in the 40th District, a bow-tie-shaped area that encompasses a sparsely settled western slice of Fairfax County as well as a largely rural slice of northern Prince William County.

Harris, director of energy and environmental programs at Rockwell International, does not seem to be particularly worried about his prospects in the Nov. 5 election. For his part, Miles says he is too busy with his regular job -- and too short of cash -- to spend as much time campaigning as he did in 1983.

In that race, Harris buried Miles by nearly a 2-to-1 margin. The final vote was 6,735 to 3,605.

Miles, a soft-spoken former Marine, Air Force pilot and navigator who served in Southeast Asia during the Vietnam War, faces a number of obstacles in the current race.

Perhaps his most serious problem is that he lives in Prince William County, a locality that includes just two of the 17 precincts in the district. A base in a rural area five miles north of Haymarket, politicians say, is a distinct liability in a race that will be decided overwhelmingly by Fairfax voters.

A second problem is a lack of funds. At a time when Northern Virginia candidates for the House can raise $25,000 or more, Miles expects to spend just $3,000 to $4,000 on his bid this year -- hardly enough to establish name recognition in a large district.

Harris says he will spend about $10,000. He claims that he uses "the very latest political techniques." He won't say exactly what those techniques are, but they do not seem to include much pressing of the flesh; Harris has eschewed door-knocking in favor of extensive polling -- although he won't say what kind or by whom. ("I can tell you what the most important issues are in my district by ZIP code," he declared.)

Miles dismisses Harris' campaign strategy as "arrogant." He added: "One of my biggest complaints about Bob is that he's aloof, he doesn't stay in touch with the voter."

He also attacks Harris as a "ghost delegate" whose record of attendance in Richmond is poor -- a charge Harris denies vehemently.

Moreover, Miles says Harris is ineffective, citing as proof Harris' ranking in a newspaper survey early this year as 75th out of 100 members of the House of Delegates. Republican incumbents -- many of whom scored low in that survey, sponsored by The Virginian-Pilot and Ledger Star -- say the poll was biased in favor of Democrats, who control the General Assembly.

"He's not that influential," said Miles. "He takes a lot of credit for things he hasn't done."

"The ultimate grade card for an incumbent," Harris countered, "is the vote on election day."

Harris further stresses that he has established himself as a member of the House Appropriations Committee, a critical assignment that positions him to deliver the goods for Northern Virginia. He cites his support for state funding for education, including George Mason University and Northern Virginia Community College, as well as his work for increased highway funds and the state subsidy for the Metro system.

"I'm offering 12 years of experience," Harris said flatly. "I'm on powerful committees. I'm in a position to bring home the bacon for Northern Virginia."