Northern Virginia may lose its last leadership position in the state General Assembly later this week when Democratic senators select their majority leader.

Facing possible ouster is Sen. Adelard L. Brault of Fairfax, 70, who in recent years has been a key figure in shepherding bills on such regional issues as Metrorail funding and approval of a George Mason University law school through a sometimes-hostile Senate.

Brault appeared to be losing ground yesterday. United Press International reported that challenger Sen. Hunter B. Andrews, 58, of hampton, apparently has picked up support from eight western Virginia senators who plan to vote as a bloc at a Democratic caucus meeting Friday.

None of the eight would comment on the UPI report. The group reportedly formulated its strategy while attending a Virginia Polytechnic Institute football game a month ago.

"We've reached an agreement and we've notified the principals," said one of the eight senators, who asked not to be identified. Andrews also refused comment.

Brault predicted last week that he would win, but said he thought the eight votes could be crucial. He did not return a reporter's telephone calls yesterday.

Andrews, who is fond of noting that his ancestors first came to Virginia in 1610, is considered more conservative, more courtly and more eloquent than the sometimes feisty and brusque Connecticut-born Brault.

"Sometimes when he's under pressure, Abe steps on people's toes," said one Northern Virginia senator who said he would support Brault. "Still, this thing [the contest for Brault's job] has taken us all a bit by surprise."

The majority leader's duties are largely administrative. He sets the senate's schedule, including deadlines for voting bills out of committee. But he also serves as spokesman for the majority, which in the Senate dominates by a 31 to 9 margin.

"I'm the most visible member," said Brault last week, noting that he was chosen in a poll earlier this year as the state's most effective senator.

Brault's ouster would mark the second time in recent weeks that a Northern Virginian lost a legislative leadership position. Arlington Del. Mary A. Marshall, who was secretary to the House Democratic caucus, was defeated in her bid to capture the caucus chairmanship from del. C. Hardaway Marks, of Hopwell. She chose not to run again for the secretary's post.

Brault, who entered the Senate in 1966, captured the majority leader's post four years ago when a moderate liberal coalition ousted the Senate's longtime conservative leadership. By a sizeable majority, the coalition fired Senate clerk Louise Lucas, a 30-year Senate employe, and stripped senior Democratic Sen. Edward E. Willey, of Richmond, of much of his power.

Neither move sat well with Senate conservatives, who have been eager for an opportunity to defeat Brault.

They saw that opportunity this year when three of Brault's 1976 supporters lost their reelection bids to Republicans.

One of the defeated was 20-year Senate veteran William B. Hopkins, of Roanoke, the recognized leader of the nine-member group west and southwest of Charlottesviille. The eight remaining western senators said yesterday they decided to vote as a bloc, in part, to compensate for the loss of Hopkins' influence.

"We decided it's an opportunity to gain a little clout for the western end of the state -- politiical clout," said Sen. Frank W. Nolen (D-Augusta).

Politically, the eight would appear to be closer to Brault. Many western Virginians share Northern Virginia's sense of resentment against Richmond, which they claim shortchanges the two regions in state aid especially for roadways and mass transit. But the senators denied they were willing to make deals with either Brault or Andrews in return for their votes.