Virginia State Sen. Adelard L. Brault, a feisty Fairfax County Democrat, was ousted today as the Senate's majority leader in what he denounced as a "power play" certain to weaken Northern Virginia's influence in the General Assembly.

Brault's defeat leaves the Washington suburbs without anyone in a leadership position to either house of the Virginia legislature.

The controversial, 70-year-old Brault, the Senate's Democratic floor leader for the past four years, was replaced in the key legislative post by Sen. Hunter B Andrews (D-Hampton). The takeover by Andrews, regarded as a prospective candidate for governer or lieutenant governor in 1981, was engineered in part by a bloc of western Virginia senators.

"I wouldn't do to my friend what some of my friends did to me," Brault said after his replacement as majority leader was announced. "It was a power play."

Brault's ouster is likely to reduce Northern Virginia's influence over such key legislative issues as financial aid for the Washington area's Metro transit system. The majority leader acts as chief spokesman for Senate Democrats, who outnumber Republicans by a margin of 31 to 9.

The shift in Senate leadership took place during a 35-minute closed-door session of the Senate's Democratic caucus, with all but two of the Democratic senators present. Recognizing that he could not win, Brault announced his withdrawal from the race for majority leader and then nominated Andrews, 58, to succeed him Andrew's election was unanimous, sources said.

Brault said later that he decided to withdraw because he considered his ouster a foregone conclusion and wanted to "eliminate as much trauma as possible" with the Democratic caucus. "When I lost the bloc of eight [Western Virginia] votes, these was no way I could win," he said.

Brault's removal was partly offset by the election of a Northern Virginia senator, Clive L. DuVal Ii of Fairfax, to the largely honorary position of Senate Democratic caucus chairman. DuVal defeated Sen. Williard J. Moody of Portsmouth in a seperate secret-ballot election. The vote was not announced. The Caucus chairman has a far less powerful position than the majority leader and, in practice, defers most decisions to the leader.

Brault's replacement was the second recent defeat for Northern Virginia legislators and left Tidewater area legislators in control of two key General Assembly posts. Del. Mary A. Marshall of Arlington was beaten recently in her bid to win the House Democratic caucus chairmanship. The recently elected House majority leader is Thomas W. Moss Jr. of Norfolk.

One factor that contributed to Brault's ouster as majority leader was the defeat by Republicans in this year's elections of three Senate Democrats who had backed Brault in 1976. Western Virginia senators were especially troubled by the loss of 20-year Senate veteran William B. Hopkins of Roanoke. The western senators agreed to vote as a bloc, partly to offset Hopkins' loss.

After his takeover, Andrews disputed Brault's charge that he lost in a "power play," praised Brault as a "very effective majority leader" and said the change will not affect Northern Virginia.

But Northern Virginia senators were chagrined. Sen. Joseph V. Gartlan Jr. of Fairfax initially tried to block the Democratic caucus from going into closed session to select its leaders. After Brault's removal, Gartlan declared, "It means the loss of a very effective voice in the Senate."

Said newly elected Sen. Richard L. Saslaw of Fairfax; It hurts an awful lot."

In uncontested races, the Democratic caucus chose two western senators for leadership posts, Virgil H. Goode Jr. of Rocky Mount as the caucus vice chairman and Daniel W. Bird Jr. of Wytheville as secretary. Sen Edward E Willey of Richmond was reelected president pro tempre.

DuVal named Sen. Lawrence Douglas Wilder of Richmond, Virginia's only black senator, as steering committee chairman. In an interview, DuVal noted that Wilder, a liberal, had supported his bid for caucus chairman.

When Brault became majority leader in 1975, Northern Virginians held leadership positions in both the Virginia House and Senate. The region's influence began to decline two years ago with the defeat of James M. Thomson of Alexandria, who was the House majority leader and then believed to be in line to become speaker of the house.

Tensions over Northern Virginia's role in the upcoming assembly session which opens in mid-January erupted later at a caucus of House Republicans. Del. Warren Barry of Fairfax accused a Roanoke County legislator of withdrawing from a face for a GOP leadership position out of fear that a Northern Virginia might win the post.

"We're not Northern Virginia Republicans, Tidewater Republicans, or Valley Republicans, we're just Republicans," Barry said at the meeting. "It's hard enough being a Republican, let alone a Northern Virginian."

His remarks angered Del. Ray. Robrecht who had withdrawn from a challenge to Del. Gerry Giesler of Hillsville as House minority leader. Robrecht accused Barry of seeking publicity and added. "If you're looking for a fight, I'll see you outside."