Northern Virginia Democrats are blasting Republic Gov. John N. Dalton this Labor Day weekend as they prepare for a Nov. 6 election that may serve as a referendum on Dalton's administration.

The governor has announced he wants the GOP to capture enough seats in the 140-member General Assembly to give him a veto-proof legislature next year.

But the Democrats, who have traditionally controlled the legislature while losing ground in recent statewide elections, already have begun making Dalton's performance a central campaign theme.

State Democratic chairman Richard Davis sent out a fund-raising letter last week to 6,000 Democrats that sharply criticizes the GOP administration for partisanship, cronyism, a state purchasing scandal and mishandling of the gasoline crisis that struck Northern Virginia this summer.

Dalton, anticipating that Democrats in the Washington suburbs might use this attack, is ready with a counterpunch: he blames President Carter and the Democrats for the gasoline problem, saying they shortchanged fuel supplies in urban areas.

The outcome of the elections will be a test of partisan strength, particularly in Northern Virginia, and a good indication for Richmond politicians of what to expect in the area.

And, perhaps more than ever, numerous special interest groups will be trying to influence local and state legislative candidates.

"I think we're going to make some gains," predicted Del. Vincent F. Calahan Jr. (R-Fairfax). "We have a better chance on the open races, especially in Northern Virginia, and we expect to maintain what we have already."

State Senate Majority Leader Adelard L. Brault (D-Fairfax) said, however, that he was just as convinced the Republicans won't make any inroads on the Democrats.

Brault says Dalton's handling of the gasoline crisis will hurt him when he comes to the area to campaign for GOP candidates. He said the governor's recent Washington television and radio interviews are "politically motivated to try to sell himself to Northern Virginians."

But the Senate majority leader doesn't discount the role of special interest groups in the election results, and his own campaign against a young conservative newcomer is a case in point.

"There is always a low turnout, and special interest groups can have a big impact on the outcome if they get their people out," said Brault, 70, who is being challenged by John Thoburn, the 22-year old son of ultraconservative Del. Robert L. Thoburn (R-Fairfax).

The young Thoburn is expected to attract the support of opponents of the Equal Rights Amendment, militant tax-cutting advocates, and other largely conservative organizations.

Officials in both parties say Thoburn will have a rough time ousting the veteran lawmaker, and Brault doesn't think anyone -- not even Dalton, who plans to campaign for Thoburn -- "can justify electing a 22-year old college student to replace the majority leader of the Senate."

Thoburn, the son, could not be reached for comment, but Thoburn, the father, warned last week that Democrats in the past have mistakenly underestimated his and his son's following.

While the Brault-Thoburn race is one of the more interesting contests, the closest is expected to be the fight to succeed retiring State Sen. Omer L. Hirst (D-Fairfax).

Two-term Del. Richard L. Saslaw (D-Fairfax), 39, a real estate salesman, is trying to advance from the House of Delegates to the Senate. He is being challenged by former Republican delegate James R. Tate, 35, an attorney and former congressional candidate.

The contest, according to one legislator, will be between "two known quantities" whose differing positions on controversial issues are clearly defined.

"I'm a conservative and he's a liberal, and there's no confusion on that," said Tate. Unlike Saslaw, he opposes the ERA and collective bargaining for public employees and advocates a constitutional amendment that would ban abortions.

While those issues and others have frequently been major themes of past campaigns in the region, Tate said he expects economic and transportation concerns to overshadow everything else this year.

"We're going back to the basic bread and butter issues this time, and that's what I'll stress," said Tate, who lost a race against Rep. Herbert E. Harris II (D-Va.) in 1976. He said he outpolled the incumbent congressman in the Senate district and is "optimistic" that he can defeat Saslaw.

Saslaw, however, said he served a year longer in the legislature than Tate and thinks he was "more effective . . . I'm running on my record."

Tate and Thoburn "are pitching to the same anti-ERA, anti-abortion crowd, and those people tend to be hardcore," Saslaw said. "But voters will want to know what he can do for Northern Virginia."

Some Democrats say they fear Tate, a Mormon, will receive extraordinary financial and staff help from other likeminded Mormons, who were particularly active in Richmond last session against the ERA.

But Tate, who stressed that politics should have nothing to do with religion, said that only a few individuals from his church are helping out in his campaign.

Northern Virginia political strategists say it will be difficult to defeat incumbents in either of the parties, although Fairfax Democrats are helpful, that their candidate can replace the Republican who is leaving the embattled sheriff's post there.

Most candidates report receiving fewer of the usual barrage of one-issue or special interest group questionnaires. However, a Virginia branch of the Gay Activists Alliance added its voting query this year to those from organizations on both sides of abortion, ERA, labor, teacher, gun control and taxpayer disputes.

"The whole world is composed of various little pressure groups, and you can survive politically if you don't go against all or a majority of them," said Sen. Wiley F. Mitchell (R-Alexandria) who is unopposed this year.

A reply of one of the most acrimonious contests four years ago is expected to be more subdued as State Sen. Joseph V. Gartlan Jr. (D-Fairfax) is again challenged by investor John B. Watkins.

The two men battled over school busing and the content of each other's campaign literature last time around and were barely civil together by election day. Gartlan won by 800 votes out of 18,000 cast.

Since then, though, Gartlan's son and Watkins' stepson have become roommates and buddies at college, and both candidates say they expect less rancor in this campaign.

"They're best friends," said Watkins, adding that the Gartlan youth "has got a really nice family."