In the dog days of summer, a political candidate's fancy turns to telephone banks and petition drives. At least it does in Northern Virginia, where an Aug. 6 deadline looms for candidates who plan to challenge members of their own party for special one-year terms in the House of Delegates.

All 100 House seats must be filled by the voters this fall, the result of a court-approved redistricting plan that divides the state into 100 legislative districts.

Even though the filing deadline is less than two weeks away, no one knows for sure which House districts in the Virginia suburbs will hold nominating primaries Sept. 7, the day after Labor Day.

"I don't know of anyone challenging an incumbent Democrat," said Fairfax Democratic Party chairman Dottie Schick. "We are just not real big on doing that, especially on challenging incumbents who are doing a good job."

Schick's GOP counterpart, Benton K. Partin, says that he knows of only three challenges to six Republican incumbents from Fairfax. "But of course it's still early, and at this point, you just don't know who's going to file at the last minute," he said.

Thus far, Partin and other Republican leaders say, delegates James H. Dillard, Robert E. Harris and John H. Rust Jr. will face primary opposition. No Democratic or Republican challengers had emerged by early this week to the other Northern Virginia House incumbents.

Dillard, a Fairfax County school official and moderate Republican who narrowly lost a primary four years ago after being targeted by the New Right, is being challenged by Gordon S. Jones of Springfield who has strong ties to the party's conservative wing.

Jones, a self-described lobbyist who worked briefly as a legislative liaison for Interior Secretary James Watt, is executive director of United Families of America, an organization headed by Sen. Jeremiah Denton (R-Ala.). Jones, who formally began his campaign last week, has received the backing of another New Right leader, Larry Pratt of Springfield.

Pratt, a former state legislator from Fairfax and executive director of Gun Owners of America, was defeated in the 1981 legislative elections, along with John S. Buckley of Vienna, a cousin of conservative columnist William F. Buckley Jr. Pratt and Buckley, both opponents of the Equal Rights Amendment, were seeking second terms and were the targets of a well-financed campaign by ERA supporters.

In a neighboring Fairfax district, former delegate Robert L. Thoburn says he has been "running since January" against Harris, a Rockwell International executive and a member of the House since 1974. Thoburn, headmaster and owner of the fundamentalist Fairfax Christian School, was defeated in 1979 after serving one term in the House.

Rust, a former attorney for Fairfax City, is being challenged by Stephen E. Gordy, a retired Army colonel and former Loudoun County school principal, whose finance chairman is John Buckley. Rust, a member of the House since 1980, was targeted by conservatives for defeat, after switching his stance on the ERA from opposition to support.

Thoburn and Jones both say they think the new single-member districts will make it easier to challenge incumbents. "This way it'll be a straight head-to-head race and the voters will have a clear choice," Jones said.

"I've been in favor of single-member districts for a long time," said Thoburn. "It makes delegates more accountable and helps candidates that are not as well known and can't afford to raise a lot of money."

Dillard said he believes single-member districts may make it easier for special interest groups such as the New Right to target incumbents. "Basically the way they worked in the past was to pick one person in each multimember district anyway and then do a lot of mailings. Now . . . instead of having to mail to a whole district, they can zero in on a relatively few precincts. It's a lot cheaper."

All three Republican incumbents agree their most formidable task will be to arouse the interest of voters, for whom an election the day after a long Labor Day weekend is about as exciting as going back to work.

"Not too many people are interested in a primary the day after a long weekend," observed Republican Del. Warren E. Barry of Springfield, who faces no announced opposition from within his party. "I guess that means whoever has the biggest family will probably win."