Northern Virginia legislators today engaged in one of the most heated partisan fights of the legislative session over a bill that could decide which party will control the Fairfax Board of Supervisors.

"It's political," said Sen. Joseph V. Gartlan Jr., a Fairfax Democrat. He said the bill was conceived to counter "a political move by the Republican Board of Supervisors to enhance the chances of reelecting Republicans."

Republican Sen. Wiley F. Mitchell Jr., who represents Alexandria and part of Fairfax and also acknowledges that the bill is political, asserted that the measure was drafted by Democrats to try to protect their own members on the board.

The heavily Democratic Senate Rules Committee, with Mitchell casting the only dissenting vote, approved the bill, which would allow local governments to redraw political boundaries only once every 10 years, after the federal census.

Under existing law, there are no restrictions on how often a local jurisdiction may redistrict itself.

Fairfax County, one of the fastest-growing counties in the state, generally has redrawn its magisterial (election) districts every five years to compensate for huge population shifts.

The board, which a year ago switched from a Democratic to Republican majority, recently proposed a new remapping to accommodate population growth since the last change in 1981.

If the bill is enacted, redistricting would be delayed until after the 1990 census.

The issue has exploded into charges and countercharges of partisan politics.

Mitchell and Republican members of the board deny that their party is trying to expand its five-member majority on the nine-member board. They argue that they are just trying to equalize the county's political districts, which range in population from 70,000 in Lee District to 104,000 in Springfield District.

Not so, say the Democrats. Republicans "just want to enhance (Republican T. Farrell) Egge's chance for reelection and defeat a Democrat," Gartlan charged after the committee hearing.

Egge won a narrow victory in the politically volatile Mount Vernon District in 1984 to give the board its first GOP majority in decades.

Mitchell said he believes the proposed law is unconstitutional because delaying redistricting would inhibit equal representation of the county's citizens. He said that if the measure passes the General Assembly, he will file a complaint with the U.S. Department of Justice.

Although the furor over the bill comes from Northern Virginia, its author, Sen. Hunter B. Andrews (D-Hampton), is from Tidewater. Today he was observed during the committee meeting walking over to two Fairfax County Democratic supervisors seated in the corner of the hearing room and asking, "Have we covered all points?"

But a few minutes later he protested to the committee that "this is a statewide bill. You all are turning it into a local bill."