This fall, more than ever before, Virginia Republicans are marshaling forces behind their candidates for the state legislature, a sign that the state GOP is starting to turn its much-vaunted machinery loose on local elections.

The GOP effort began last January with a novel $50-a-ticket fund-raiser in Richmond sponsored by the General Assembly's Republican caucus. It has been sustained by the Virginia Victory Fund, a new committee that is steering GOP donors to the bottom, as well as the top, of the ticket. And it is being capped by the Republican National Committee's $200,000 media campaign pushing themes designed to help GOP candidates from statehouse to courthouse.

With these initiatives, Republicans hope to capitalize on changes in Virginia's legislative districts and increase their minority of 25 in the 100-member House of Delegates by at least four -- maybe as many as nine -- seats. They see their most likely gains in areas of new suburban growth, including the Northern Virginia counties of Fairfax, Loudoun and Prince William.

The goals are modest for a party that has held the governor's office for the last 12 years and last year won nine out of the state's 10 congressional seats. And with only 58 candidates running for 100 seats, Republicans are still a long way away from challenging the Democrats' supremacy in the General Assembly, a legacy from the days when Virginia was a one-party state.

But if J. Marshall Coleman is elected governor next month, a gain of nine seats for Republicans would presumably give him a "veto-proof" legislature, or enough votes to block the two-thirds majority required for an override. At least that is the pitch made to contributors who were asked at a Victory Fund party at the governor's mansion Sept. 22 to make pledges -- some for the first time -- to local political races.

The Republican effort on behalf of legislative candidates is hardly prodigious, at least compared to New Jersey, the only other state to hold elections this year. There the Republican commitment to legislative races is costing $1.7 million. In contrast, direct state and national contributions to legislative candidates in Virginia are not expected to exceed $50,000.

But any effort at all, however small, is seen by party members as a sign of GOP maturity in Virginia. "If we are going to achieve the goal of becoming a majority party in this state, we're going to have to do it by winning seats in the legislature," said Ed Stikes, state GOP executive director. To John Marsh, a Prince William County horse breeder who heads the Victory Fund, the new emphasis on the legislature, while not as "romantic" as presidential or gubernatorial races, shows that the party has "come of age."

By zeroing in on so-called "areas of opportunity," Republicans say they are showing a new sophistication. "The temptation of the past was to spread a little out to everybody," said Del. David Speck (R-Alexandria). "This time, we are making tough decisions about races where the money will make the difference."

The exact number of targeted districts is unclear; Speck estimated that 20 candidates, including all the joint slates running in Northern Virginia, are receiving "substantial assistance," or more than $500 per candidate, $1,000 for joint campaigns in multi-member districts. According to Stikes, the number of targets continues to change, depending on the money available.

For the most part, the Republicans' opportunities in 1981 came about because of the new district lines drawn by the legislature last session. Although that plan has been declared unconstitutional by a federal court on grounds that it violates one-man, one-vote guidelines and new elections have been ordered for next year, redistricting has given the GOP a chance to single out areas of new population growth where they have traditionally done best.

Analysts differ, but most agree that Republicans this year could make gains in Fairfax, Virginia Beach and Chesterfield where five new seats have been created. In Fairfax, where Republican incumbents already outnumber Democrats seven to two, help from the state party is amplified by $1,000 contributions to each delegate candidate by the county GOP. In Chesterfield, Democrats have conceded a seat by running only two candidates in a three-member district.

Special attention has also focused on the new single-member district in Loudoun where incumbent Democrat Earl E. Bell is up against Kenneth B. Rollins, one of seven Virginia candidates who recently got a presidential blessing from Ronald Reagan. In other districts, including Prince William and Arlington, candidates have been given money by the state party for joint brochures and other expenses.

In starting their legislative campaign this year, the Republicans took a page from the Democrats' book and invited lobbyists to a fund raising at the John Marshall hotel in Richmond on Jan. 28. "It was the first time we'd ever done this," said Delegate Vincent Callahan (R-Fairfax). "We decided as a caucus to get started and raise some money ourselves, something the Democrats had been doing for years."

Not all Democrats are impressed with the Republicans' efforts this year. Del. Alson H. Smith, a veteran Democratic fund-raiser from Winchester, scoffs at their Richmond fund-raiser -- "they didn't raise enough to buy a billboard for each other" -- and stresses the down-home strength of most Democratic incumbents.

"The legislature is one of the most respected groups in Virginia. All our polls show that," said Smith. "After all, they the legislators wouldn't be put there if they didn't have the political philosophy of their constituencies."

But privately some Democrats foresee a possible loss of four seats to the Republicans this fall which, they concede, is a testament to GOP strength in the state's growth areas.