A gray Cadillac, with a "Delegate 48" license plate, sits in the driveway of the two-story brick house, the home of Virginia Del. Robert E. Harris.

A few hundred yards away another two-story brick house overlooks a similarly quiet Fairfax County dead-end street--home of Harris's fellow Republican Del. James H. Dillard.

The two homes, separated by trees, a few houses and the rolling Olley Lane southeast of Fairfax City, will soon also be separated by an invisible precinct line dedicated to the preservation of Dillard's and Harris's jobs.

The line is the critical link on a map that a few months ago many politicians said could not be drawn: a plan carving Northern Virginia into 21 legislative districts without carving any legislators out of their seats.

To protect all the incumbents, the delegates had to cut through an existing precinct and create one district that sweeps impressively from Langley into Loudoun County. They also cut across Arlington, Alexandria and Prince William County lines and state senate, supervisor and congressional boundaries--creating so many new districts that every four years the Fairfax voting registrar will have to prepare 30 different ballot combinations for voters in the county.

Although the comprehensive state plan that contained the Northern Virginia single-member districts was vetoed by Gov. John N. Dalton last week, a similar single-member plan is expected to remain in any redistricting proposal that is finally approved by the Virginia General Assembly, the governor, the courts and the Justice Department.

If the map caused problems for the registrar--and troubled some prospective candidates who were tucked into hard-to-win districts--it satisfied most incumbents.

"Somewhere there is a balance between not 'gerrymandering' to protect incumbents, and not arbitrarily putting incumbents out of office," said Democratic Del.-elect Vivian E. Watts, who devised the Fairfax plan with Republican Del. John H. Rust Jr. To resolve the question of how to keep all 12 Fairfax delegates in the county without having to pit any incumbent against another in the upcoming November elections, the two worked through the night and early into one December morning counting precincts and drawing lines in the map room of the State Capitol in Richmond. "After all, the people of Fairfax did elect these people."

The Fairfax delegates protected each other with a bipartisan spirit that contrasted with other parts of the state and baffled some Democrats, who hold a 75-to-25 edge in the House. "That's crazy, not to put them (Harris and Dillard) in the same district," said Del. Warren G. Stambaugh, an Arlington Democrat. "We would have done it. The Democrats in the General Assembly would have gladly done it."

But Republican legislators outnumber Democrats in the Fairfax delegation, and the way the Assembly operates, sectional politics are allowed to outweigh partisanship. "We're Northern Virginians and we have to work together," said Republican Del. Warren E. Barry. "Partisanism takes a second seat."

In a dozen special sessions this year that have cost more than $1 million, the House of Delegates has failed to draw a statewide redistricting map that the courts, the Justice Department and Dalton will all accept. Dalton's veto of the most recent House plan once again placed the process in limbo one month before a court-ordered deadline to produce an acceptable plan.

Dalton's major objections to the most recent plan centered on the multi-member districts in some of Virginia's largest cities. Civil rights groups say that those districts would dilute black voting strength. The current Northern Virginia plan cuts through large districts in Fairfax, Alexandria, Arlington and Prince William now represented by two or three delegates and creates smaller districts that would elect only one delegate each. The danger of such a plan to incumbents was alarmingly illustrated in November when the staff of the House Privileges and Elections Committee drew a map that threw four incumbents into one district, three into another and two into a third, while creating six new districts with no incumbents at all. The incumbents would have none of it.

"The saying was going around that P & E wanted to get Fairfax's attention," Rust said. "I think they did."

Rust, Watts and others took the map into their own hands. Watts, who with Harris, Dillard and Del.-elect Gwendalyn Cody, lives in a kind of legislator ghetto in central Fairfax, had drawn a map last spring that put Harris and Dillard into the same district. Harris and Dillard immediately indicated they didn't like the idea, and they were supported by Del. Vincent F. Callahan Jr., the senior Republican of the Fairfax delegation and a member of the elections committee.

So Rust, Watts and Dillard drew a line up Olley Lane and across Long Branch Creek, splitting one precinct but threading between the two delegates' homes. The lines left Dillard, Watts, Harris and Cody each in separate districts, though they live within two miles of each other.

"Wasn't that clever?" said M.C. Rappleyea Sr., secretary of the Fairfax Electoral Board and the man who must draw new precincts once the delegate lines are approved.

Callahan had a problem of his own in McLean, where he and newly elected Republican Robert T. Andrews both live. To avoid pitting the two Republicans against one another next year, Andrews' Langley precinct was tucked onto a riverfront district that stretches across northern Fairfax County, leaving Callahan a compact district in the heart of McLean.

"I think Andrews is probably less happy than some others," said Dillard. "After all he's a new boy on the block, and he was looked after as well as can be expected."

The move displeased Democratic Sen. Charles L. Waddell of Loudoun, since his county will be part of two separate districts instead of having a delegate of its own. Waddell called the plan a "travesty."

"It's to protect the incumbents elected in 1981," Waddell said. "I understand the purpose, but I don't like it."

Two Fairfax delegates associated with the New Right who were turned out of office this fall may be equally displeased. John S. Buckley and Lawrence Pratt, both of whom pledged on election night to run again in 1982, now may have to oppose strong incumbents from their own Republican party, Rust and Barry.

"When you come to somebody like Larry Pratt and Buckley, you've got a wider berth in ideology," Barry said. "They (the Democratic incumbents) don't mind stepping on those toes."

Pratt, who lives in what would be Barry's district, said he is weighing his 1982 options. Buckley could not be reached for comment.

The Democrats initially juggled some precincts in the Route 1 corridor to make the district of newly elected Republican Frank Medico more Democratic, possibly giving a leg up to defeated 1981 Democrat David L. Temple Jr. Barry challenged the move on the House floor, and the Democrats--either unwilling to defend the boundary or acknowledging that Democratic incumbent Gladys Keating would be happy to have the Democratic precincts back--acquiesced.