The conservative drift of policies in Virginia, already one of the most Republican-dominated states, is likely to be accelerated dramatically by changes dictated by the 1980 census.

Analysts say that by the end of the decade, Republicans may be in control of at least one house in the state legislature, Virginia's last major bastion of Democratic political power. If so, it would mark an end to more than a century of Democratic Party dominance of both houses of the 140-member General Assembly.

"The Democrats will get slowly eaten up," said Democratic political strategist Paul Goldman."They're looking at political erosion."

These projected losses are the result of the continuing explosive growth of Virginia's suburbs, which have become the electoral bedrock of conservative Republicanism in the state. The GOP's likely gains there, the analysts say, will come at the expense of traditionally Democratic cities and rural areas, which lost population in the census.

Arlington, Alexandria, Richmond and Norfolk combined will lose four or five House of Delegates seats -- all of them currently Democratic -- in this year's constitutionally mandated reapportionment. The seats will fall to increasingly Republican areas such as Fairfax County, Virginia Beach and the suburbs of Richmond.

"When you combine the population trends with the state's underlying Republican trend, it's an inevitability that the Republicans will take over the legislature," says University of Virginia political scientist Larry Sabato.

Goldman, who engineered former lieutenant governor Henry E. Howell's upset victory in the state's 1977 Democratic gubernatorial primary, also believes reapportionment could destroy the legislature's small vestiges of liberalism, wiping out at least half the state's four black delegates and removing the "floater" delegate's seat shared by Arlington and Alexandria that has traditionally been one of the state's most liberal.

Ironically, the redistricting that will aid the GOP will be hammered out this spring by a legislature still dominated 3 to 1 by Democrats. But most of the lawmakers contend the one-man, one-vote strictures dictated by the Supreme Court in 1963 make it impossible to gerrymander legislative districts to the Democrats' advantage.

"We'll do it [reapportionment] in compliance with the Constitution and the law and we'll do it fairly and objectively," says Senate Majority Leader Hunter B. Andrews, who has assured himself of tight control over the process by naming himself chairman of both of the Senate's redistricting subcommittees.

In the past Virginia Democrats largely have treated the reapportionment process as an "incumbents' survival act," according to Sabato. But he believes that recent Republican gains in Congress -- the GOP now holds nine of Virginia's 10 House of Representatives seats -- plus the likelihood of future legislative inroads will put enormous pressure on Democrats to make this year's reapportionment a highly partisan affair.

Already, a number of Democrats are talking about redesigning the Southside Virginia district represented by Republican Eva Scott of Amelia, the Senate's only female member, to cause her defeat. A major political struggle also is brewing among Northern Virginia lawmakers over the future boundaries of the region's 8th and 10th congressional districts.

Both seats had been held by Democratic incumbents for six years until Ronald Reagan's landslide helped oust them. As a result, a substantial group of Democratic lawmakers in Arlington and Alexandria would like to see their cities combined along with the inner-Washington Beltway portion of Fairfax to make an urban-dominated 10th District seat that Democrats would have a good shot at winning and holding.

"That kind of district would be compact and it would combine people of like interests and that should be the basic criteria," says Arlington Del. Warren G. Stambaugh, who readily concedes the redesign could also lead to the defeat of Republican Rep. Frank Wolf.

But Democrats from the outer suburbs of Fairfax County, which gave Reagan an almost 2-to-1 majority last fall, have no desire to be redrawn into an 8th District stripped of its urban Alexandria Democratic base. State Sen. Adelard L. Brault, dean of the county's legislative delegation, is one of many opposed to such a move.

Republicans already dominate the county's House delegation, holding eight of 10 seats. They also believe they can take the two additional seats that the county should pick up this year. "Our future looks very good," says Fairfax Del. Vincent F. Callahan, Republican legislative caucus chief.

The GOP also hopes to begin making inroads to Fairfax's five Senate seats, all held by Democrats, and to win the extra seat the county may gain by reapportionment. Some Fairfax Democrats, most notably Sen. Clive L. DuVal, who may lose a large chunck of his liberal Arlington constituency for the more conservative-minded voters west of the Capital Beltway, say they are concerned.

"That's Indian country for some of us, but I think once they get to know me and my legislative record, I'll get their votes," says DuVal, a McLean lawyer.

Other Democrats dispute claims that their party is in real danger in the legislature. "I'm not one bit at all pessimistic," says Sen. Richard Saslaw, who won a hard-fought campaign in 1979 in a conservative Fairfax district."As long as the party puts up candidates who work hard and are perceived as ideoligically similar to the voters they represent, they'll win. You're talking to one who did."

Many analysts believe Richmond and Norfolk's three black delegates are in political trouble because they serve in multimember districts whose representation will be reduced due to population losses. One cure might be to carve both cities into single-member districts so that blacks could run in predominantly black neighborhoods. But many white incumbents are likely to reject that idea because they believe they are protected from aggressive challengers in larger districts.

Sabato said he believed President Carter's Justice Department, which under the Voting Rights Act must approve the reapportionment, might reject multimember districts in the state because they could weaken black representation. "I don't know what Reagan's people will do but it makes the possibility of a rejection much less likely."