Virginia Republicans say they expect to make modest gains in the House of Delegates in Tuesday's elections regardless of whether their party's nominees for statewide offices win.

The GOP, which holds 34 of the 100 House seats, hopes to pick up three or four of the 41 seats that will be at stake in the elections, party officials say. Candidates seeking the other 59 seats, most of them Democratic incumbents, are unopposed.

Many of the contested races are in Northern Virginia, where the GOP has targeted four of the eight seats that party officials say they have a a chance to win. The Republicans say they hope to win an open seat in Prince William County and defeat state Del. Bernard S. Cohen of Alexandria, Gladys B. Keating of Fairfax County and David G. Brickley of Prince William.

State Democratic officials acknowledge that the GOP is offering strong challenges in some legislative races despite newspaper polls that show Gerald L. Baliles, their nominee for governor, running ahead of his Republican opponent, Wyatt B. Durrette. "Holding down a major shift is my major goal," said Bobby Watson, the state Democratic chairman. "I'd be elated to win some . . . but we do have a comfortable majority at this point."

In 1981, despite Gov. Charles S. Robb's election as the first Democratic governor of Virginia since 1965, Republicans made dramatic gains in the House, increasing their representation there to 33 delegates from 25. The party won one more seat in a special 1982 election but there was no net shift in party alignment in the 1983 elections.

"Legislative elections are insulated from what's going on at the top of the ticket," said Thomas R. Morris, associate professor of political science at the University of Richmond. "I would not expect to see any coattail effect."

Morris also said that the Republican gains will probably not be large enough either to threaten the Democratic dominance in the House or to position the GOP to overturn the Democratic majority in the 1991 House elections following redistricting.

"If they got 10 or 12 [new] seats, that is a great victory putting them in easy striking distance of a majority," Morris said. "But the odds are you're not going to have that many upsets."

Republican State Chairman Donald W. Huffman said the GOP's drive to capture seats in the House should produce some new delegates, but that Robb's popularity will probably prevent any large Republican gains.

The Democrats "have much more weapons at their disposal when they have the governor," he said. "It's extremely difficult for us to make gains when they have the governor."

Republicans are banking their hopes this fall in part on "the Bagley seats" -- the two open seats that were left vacant by the retirement of Floyd C. Bagley of Prince William and Richard M. Bagley of Hampton (no relation).

The retirement of Prince William's Bagley, who served in Richmond for 10 years, has Republican John A. Rollison III, a Woodbridge businessman, running against George C. Dowd, a lawyer with Bagley's Dumfries law firm.

In Alexandria's 46th District, Republican Anne H. Moore, a former congressional aide, has amassed a large campaign war chest, more than $40,000, in her bid to unseat Cohen, a lawyer who has been in the Assembly for five years.

Republican Edith L. Stratton, a GOP activist, is challenging Democratic five-term incumbent Del. Gladys B. Keating of Fairfax's 43rd District in another of most expensive contests in the state.

In eastern Prince William's 51st District, Republicans are hoping lawyer Mark A. Moorstein of Manassas can beat Democrat David G. Brickley of Dale City, a member of the House since 1976.

Northern Virginia Democrats say the most vulnerable Republican incumbent is Del. Gwendalyn F. Cody of Annandale, who is fighting Democrat Leslie A. Byrne in a race that has focused on Cody's ranking by a Norfolk newspaper survey as the least effective member of the House. Byrne has benefited from volunteers and money from the state party.

One measure of the GOP strength in Fairfax County is the fact that none of the county's four Democratic delegates is without a challenger. But of Fairfax's seven Republican delegates, five -- House Minority Leader Vincent F. Callahan of McLean, James H. Dillard of Fairfax, Robert K. Cunningham of Springfield and Frank Medico of the Mount Vernon area -- are not being challenged. Neither are Arlington Democrat Mary A. Marshall and Manassas Republican Harry J. Parrish.

Elsewhere in the state, Republicans expect to retain the Virginia Beach seat vacated by W.R. (Buster) O'Brien, the party's candidate for state attorney general.