Democratic challenger John Flannery has dogged Republican Rep. Frank R. Wolf through Northern Virginia this summer, popping up at Wolf's coffees and town meetings, sometimes to pepper his opponent with questions, other times to pass the flak to others.

Last week, Wolf, a two-term incumbent, went on the offensive, accusing Flannery, a former federal prosecutor and relative newcomer to Northern Virginia politics, of having "distortions and half-truths" in his campaign literature.

Thursday evening, Flannery acknowledged in an interview that one of his claims about Wolf was inaccurate and dispatched a letter of apology to the congressman.

That pre-Labor Day exchange may be a precursor, many politicans say, to the intensive campaigning that has become a hallmark of Northern Virginia's two congressional districts. The two races, contests that traditionally begin on the holiday, this year are well under way in both districts.

In the volatile 8th Congressional District, Democratic challenger Richard L. Saslaw has attacked Republican Rep. Stan Parris for requesting that all questions for their upcoming debate be submitted to him three days in advance. Saslaw, a state senator from Fairfax County, has charged that Parris, who is seeking his fourth term, improperly used taxpayers' money to mail two newsletters proclaiming his accomplishments.

The Parris camp says the mailings were legal and that Saslaw, in the words of a Parris aide, is "acting desperate." "Even Saslaw's own polls show we're doing quite well," said Parris, referring to a poll Saslaw released last month showing Saslaw trailing Parris by almost 20 percentage points.

Still, the Parris-Saslaw race promises to be the most expensive congressional contest in the Washington area this fall. Parris, 54, a former automobile dealer, has amassed a war chest of about $550,000 and estimates that his campaign will cost about the same as two years ago, when he spent about $750,000.

Saslaw has considerably more modest goals, as do Wolf and Flannery. Saslaw says he hopes to raise about $400,000 and has collected about $130,000.

Wolf says he has raised about $300,000 and estimates he will need to raise $200,000 more. Flannery has raised about $100,000 and estimates he will need $300,000 to $500,000.

Flannery, 38, making his first bid for elective office, is focusing on national issues, particularly the nuclear arms race, while Saslaw, 44, is concentrating on local issues such as transportation. Saslaw is stressing his local ties -- he owns six service stations and has represented southern Fairfax County in the General Assembly for nine years.

Flannery, who moved from New York to Arlington in 1981, is touting his experience as a federal prosecutor and a U.S. Senate staff member.

He has a flashy campaign style -- he frequently refers to his campaign as "a coalition of pains and dreams" -- that supporters hope will favorably contrast to Wolf's style, which his supporters acknowledge is often bland.

Democrats and Republicans generally say that the closer race may be in the 8th District, which includes Alexandria and southern Fairfax, eastern Prince William and northern Stafford counties.

Parris, a former Fairfax County supervisor, was elected as the area's congressman in 1972 and was defeated two years later by Democrat Herbert E. Harris. Parris ousted Harris in 1980 in the Reagan landslide and won reelection in 1982, both times by a margin of about 1,000 votes.

The State Elections Board last week placed independent Donald W. Cap of Alexandria on the ballot there.

Flannery's battle against Wolf, who won his seat in 1980 after two defeats, is considered a tough, uphill fight. The 10th District includes northern Fairfax plus Arlington and Loudoun counties as well as the cities of Fairfax and Falls Church.

"Wolf is absolutely safe. He's worked hard," said Fairfax County Supervisor Thomas M. Davis, a Republican.

"It's a tough one," says one Northern Virginia Democratic official, who asked not to be named. "Frank has worked well with the local issues and constituent services."

Davis and others say that Wolf, 45, has been visible on local issues because of his positions on the House Post Office and Civil Service Committee and the Public Works and Transportation Committee. About 25 to 30 percent of the potential voters in Northern Virginia work for the federal government, and polls show that the number one local issue is transportation.

Parris says he has been hard-working and aggressive, and he points to a wide variety of issues with which he has been involved. The Parris camp predicts he may carry Alexandria, traditionally a Democratic stronghold, this year.

Alexandria Mayor Charles E. Beatley Jr., a Democrat and a Saslaw supporter, says he expects the Democrats will do well in the city, but he acknowledges that Parris has support there. "Stan does jump when you call him," Beatley says.

Wolf and Parris supporters expect two other Republicans on the fall ballot, President Reagan and Sen. John W. Warner, to do well and to help their efforts, especially among conservative Democrats.

A Parris spokesman said he thinks the congressman will get support from groups that traditionally support Democrats, particularly Jewish organizations. "Although Saslaw is Jewish, Stan has been a very good friend to Israel," the spokesman said.

"That's outrageous," said Saslaw. "Stan Parris has one of the worst ratings of any congressman with AIPAC the American Israel Public Affairs Committee ."

AIPAC does not have ratings, but Parris' record on issues on which AIPAC has been active is generally favorable for the group. Parris supported efforts to move the U.S. embassy from Tel Aviv to Jerusalem, to ban further arms sales to Jordan until the country agrees to negotiate with Israel, and to support Israel's efforts to build a new fighter bomber.

Saslaw has been pounding the pavement for several months and estimates that, combined with his door-to-door efforts in his state race last year, he will have hit one out of every three households in the district by election day. Parris strategists, however, are quick to point out that Saslaw cannot win with one-third of the votes.

Parris believes that his efforts on local issues -- from fighting problems connected with the District-run Lorton prison to opposing a public housing project in Springfield to trying to rid the Potomac River of hydrilla -- will reelect him.

Saslaw has attacked Parris, saying that he has not gotten a single bill passed and that he has been ineffective on civil service issues -- crucial to local federal workers -- because he is not on the House Post Office and Civil Service Committee.

Parris says Saslaw's comments "shows a real lack of understanding of how things work" in Congress. For instance, said Parris, the House recently passed a provision of his requiring the Army Corps of Engineers to work to eliminate hydrilla from the Potomac River. That provision was accomplished not by a bill but by an amendment, he notes.

A Parris spokesman says that Parris is not on the civil service committee because Wolf wanted that assignment and the Republican leadership would not put both Northern Virginia congressmen on the same committee.

He noted that Parris is on the District of Columbia Committee, which tackles issues involving Lorton and the Potomac.

Flannery says the 10th District needs a congressman who "can do both" local and national issues. He says he would seek a seat on the House Armed Services Committee because he believes that the nuclear arms race is the most important issue facing the nation.

Some Democrats say that if Flannery hopes to win he cannot afford to make mistakes, and they cringe over two of his problems last week. Flannery's literature said that Wolf was asked by a television reporter if he ever said the United States could win a nuclear war. The flyer said " . . . Wolf said, 'I don't think I said that,' He doesn't know. Isn't that incredible?"

After reviewing a tape of the broadcast, Flannery sent Wolf a letter of apology, agreeing that Wolf was incorrectly quoted. "I still say that Frank Wolf acts like he thinks we can win a nuclear war, even if he never said that," Flannery said.

Separately, Flannery accused Wolf of failing to vote in subcommittee for a study of pay equity for women in the federal government. The Democrat acknowledged Thursday that Wolf is not a member of the subcommittee and that, when the House voted on the measure, Wolf supported the study.

While some Republicans, including Fairfax's Davis, are optimistic that Wolf could get more than 60 percent of the district's vote Nov. 6, Wolf remains characteristically cautious.

"Who's saying that?" he said increduously. "This has always been a close district. You know, I lost in 1976, and in 1978 . . . . " He shook his head and shrugged.