The four liberal Democrats who represent Arlington in the conservative House of Delegates here have often fought -- and lost -- together, from struggling to repeal Virginia's sales tax on food to trying to tighten the state's admittedly weak handgun laws.

But this year's legislative reapportionment has caused that unity to crumble. Because their urban district lost population throughout the 1970s to the rapidly growing and increasingly conservative outer suburbs, the Arlington delegates are facing a political game of musical chairs -- there are four of them and only three seats to fill.

And since no one appears willing to drop out, the four are preparing for a primary battle that is already turning bitter. Three of the incumbents -- Mary Marshall, Warren G. Stambaugh and James F. Almand -- are readying a joint campaign, which others contend is an effort to intimidate the fourth, Elise B. Heniz.

"This is something we've been thinking about and worrying about for years," says Marshall, the senior Arlington member. "Now it's here and it's awful."

What is happening to the Arlington Four is certain to be repeated for liberals and Democrats around the country this year as the older urban areas that have been their political strongholds lose power in Congress and state legislatures due to declining population. It is particularly painful for the Arlingtonians, who have played the role of underdogs in Richmond for so long.

"We do a lot of swimming upstream here," says Arlington Sen. Edward Holland, a close political ally of all four delegates. "So we've had to work together. It will be even harder when there's one less person to help, but we have no choice . . . The numbers just aren't there."

The numbers Holland refers to are from the 1980 census. They show that Arlington and its neighboring city alexandria lost nearly 40,000 persons during the 1970s while neighboring suburb an Fairfax County gained 140,000.

Under the state's 1971 reapportionment scheme, Arlington had three seats of its own and Alexandria two; both jurisdictions shared a sixth "floater" seat. It is that seat, which Heinz, an Arlington lawyer, has held for four years, that is being abolished by this year's redistricting plan.

Supporters of Marshall, 59, Stambaugh, 36, and Almand, 32, had hoped that Heinz, 46, would submit to the inevitable, graciously bow out and wait for another political opportunity to come along. In the meantime, the three plan to announce their joint candidacies together soon after the General Assembly's current reapportionment session adjourns.

"They're beginning to look like the Gang of Three," says Marianne Fowler, chairman of the Virginia Women's Political Caucus, a feminist group that strongly supports Heinz.

Heinz, who says she has just as much right to one of the remaining seats as the other three, has no intention of retiring quietly. She has firmly shot down suggestions that she become a state judge or run for another office.

"I don't want to sit up on the bench in a black nightie and rule on the same case over and over again," says Heinz, who says she hopes her fellow Democrats will agree to a "friendly" primary.

Others who have been through Northern Virginia primaries say there is no such thing as a friendly one. "I think it's going to be very nasty and I'm not sure Elise, not having run into this before, knows what she's getting into," says Holland.

The scene of this feud is an urban area that, until recent years, had been regarded as Virginia's most liberal community, a reputation it earned as a bedroom community for New Deal bureaucrats who came to Washington to work for Franklin Roosevelt.

The GOP, which now boasts a 3 to 2 majority on the County Board and last fall captured the area's congressional seat from liberal Arlington Democrat Joseph Fisher, is expected to make a tough run at the three survivors of the Democratic primary.

"The Republicans are very interested in these seats and if we get too divided over this, they could get some of them," says Sharon Davis, a local Democratic activist and party official.

No one has publicly criticized Heinz for staying in. But Stambaugh and other Arlington Democrats privately have warned some of Heinz's supporters that she faces political oblivion if she runs in the primary and loses.

Marshall, a 13-year legislative veteran with strong ties to community and senior citizens groups, is expected by all to lead the ticket in a primary, as she has done in general elections for a decade. Stambaugh, the next most senior member and a longtime leader of efforts to repeal the state's 4 percent food tax, is considered by many to be Northern Virginia's effective delagate, but he admits he is running scared.

"I been on the cutting edge of some hot issues here and I'm paranoid about every election," says Stambaugh. "I guess it's possible to have a friendly primary but how realistic it is is another matter. . . If being friendly means losing my seat, I'm not going to be friendly."

The delegate generally believed most vulnerable to Heinz's challenge is Almand, a soft-spoken former prosecutor who was elected to the House in 1977, the same year as Heinz. Almand, who has built a reputation as a quiet but hard-working advocate for such issues as criminal justice reform and tenants' rights, says simply, "Nobody likes the situation . . . but I intend to run for my seat."

What is happening in Arlington also will occur in Richmond and Norfolk, where solid Democratic House delegations must also shrink. Those most threatened include some of the assembly's few black and women members.

"Of the nine women here, seven are elected out of urban areas and they're the ones that have lost in population," says Fowler of the Virginia Women's Political Caucus, who calls Heinz's reelection "absolutely critical to us and our legislative plans."

Whoever wins, says Fowler, "It's a bitter pill to have to deal with. I would have done just about anything to prevent this from happening."