A powerful legislative committee chairman is threatening to strip Fairfax County of one of its prospective two new seats in the House of Delegates, a move that could touch off a major regional power struggle over legislative redistricting.

Del. John Gray (D-Hampton), chairman of the House Privileges and Elections Committee that oversees this year's constitutionally mandated reapportionment, has told the panel's Northern Virginia members that he believes the region is entitled to only 20 House seats, one fewer than the number that suburban Washington lawmakers believe 1980 Census figures entitle them to.

The new dispute is the first in what many lawmakers believe will be a series of acrimonious battles over reapportionment triggered by the growing strength of the Republican Party and by regional rivalries. The last reapportionment 10 years ago resulted in at least two federal lawsuits, including an unsuccessful effort by Fairfax lawmakers to retrieve a "lost" delegate.

The seat Gray plans to scrap this time also would come from the Republican-dominated Fairfax delegation, which some lawmakers had expected to expand from its present 10 to 12 members under the redistricting. Gray wants to hold Fairfax, the state's most populous jurisdiction, to 11 delegates, a move Republicans say would be a blatantly political attempt to deny the GOP another seat in the Democrat-controlled House.

"The effect will be to cut down on Republicans -- they stole one from us last time and now they're going to try to do it again," said Fairfax Republican Del. Lawrence Pratt, referring to the 1971 redistricting plan in which the county was denied a seat that ultimately went to the Democrat-dominated Norfolk area.

So confident have Northern Virginia legislators been of receiving 12 Fairfax House seats for this fall's elections that they have already drawn up a plan to divide the county into four three-member districts.

Fairfax delegates have vowed to file suit again this year to overturn such a plan if it should be adopted by the legislature. And Republican Del. Robert E. Harris said he would call on the Justice Department, which he noted is now run by a Republican administration and which under the Voting Rights Act must review and approve Virginia's reapportionment, to reject such a redistricting plan.

Northern Virginia Democrats say they, too, would fight against losing another advocate of regional interests. "As a Democrat, the idea of having an extra Republican out in Fairfax doesn't help me sleep at night, but from a regional point of view we need every delegate we can get," said Del. Warren G. Stambaugh of Arlington.

Gray denied today the he had reached any final conclusion about the size of the Fairfax delegation, but said census figures indicate Fairfax is likely to fall short of the population needed for a 12th seat. He also denied politics played any role in his calculations.

"This will be decided by full committee, not by me alone, and it will be decided by the (population) numbers," said Gray. The assembly will reconvene in the early spring in a special reapportionment session.

Preliminary figures indicate the each of the state's 100 House seats should represent about 53,200 people under the one-man, one-vote doctrine established by the Supreme Court. Northern Virginia lawmakers say their region, with a population of 1.1 million, should be entitled to 21 seats -- 3 in Arlington, 2 in Alexandria, 3 in Prince William County, 1 in Loudoun County and 12 in the two Fairfax County districts that include Fairfax City and Falls Church.

But Gray has said he wants to take Falls Church and its 9,500 people out of a Fairfax district and add it to Arlington's House district. That would make Arlington a near "perfect" three-member district of 161,000, but would deny Fairfax some of the population it needs to qualify for a 12th seat.

Area lawmakers argue that Falls Church, a city that was carved out of Fairfax in 1950 and still shares its court system with the county, logically belongs in a legislative district with Fairfax. The also note that Fairfax, whose population of 595,500 is more than twice that of any other Virginia locality, is one of the state's fastest-growing areas. New residents will be denied proper representation in the General Assembly if the extra seat is lost, they contend.

"The bottom line is we're entitled to 12 seats and we're going to fight for them," said Del. Vincent F. Callahan (R-Fairfax), the senior Northern Virginian on the Privileges and Elections Committee.

Gray is considered one of the key members of the inner circle of conservative downstate lawmakers who rule the House, but it was unclear today whether his proposal has the support of the House leadership. Del. Claude Anderson (D-Buckingham), another influential downstater and a senior member of the committee, said he and other members would not begin to consider the issue until the present legislative session ends.

The General Assembly plans to hold a special session at the end of next month to vote on redistricting. The committee will hold a series of public hearings around the state in three weeks and is expected to hammer out boundary lines for reapportionment after the hearings. But many lawmakers believe Gray already has determined where many of the lines will go and will attempt to pressure committee members into supporting him.