The Virginia Senate today set the stage for the first General Assembly floor debate on the merits of the Equal Rights Amendment by voting, 21 to 17, to take the proposal out of the hands of the committee that has considered it for four years.

The controversial constitutional amendment, proposed by Congress in 1972 and ratified by 35 state legislatures, has never been reported to the floor of the Virginia House of Delegates and has made it to the floor of the Senate only once before. On that occasion, in 1975, ERA was sent back to the Senate Privileges and Elections Committee on a motion to recommit it.

Only three more states must ratify the amendment, which would ban discrimination based on sex, before March 22, 1979, to make it a part of the U.S. Constitution.

Virginia has not been listed by national ERA lobbyists as one of the states likely to ratify, but state lobbysists for the amendment were jubilant over today's vote in the Senate.

"We are hoping that a favorable vote in the Senate will put a lot of pressure on the House to act on it," chief ERA lobbyist Norma Murdoch-Kitt of Richmond said in an interview.

Del. Dorothy S. McDiarmid (D-Fairfax), one of seven women in the 140-member General Assembly and an ardent ERA supporter, said she thinks favorable Senate action could change some votes on the House Privileges and Elections Committee, which has killed House consideration of the measure in the past.

Counts by both supporters and foes show that committee to be opposed to ERA, 12 to 8. An effort last Friday to change the House rules to bypass the committee failed, 62 to 36.

The size of the majority against putting ERA before the full House was a major disappointment to its backers. Despite their enthusiasm over today's Senate vote, most legislators who have followed the issue closely believe chances are slim that either the House Privileges and Elections Committee or the full House would approve the amendment this year.

"I don't think it's going to be ratified." Sen. Adelard L. Brault (D-Fairfax), an ERA supporter and the Senate majority leader said today.

Brault's support of the motion by Senate Privileges Elections committee chairman Omer L. Hirst (D-Fairfax) to discharge the committee from consideration of ERA was crucial. Last year, when Hirst made a similar motion, Brault opposed it and was blamed by ERA backers for its defeat.

Today, Brault said he supported discharge because, unlike last year, the committee has not already taken a negative vote on ERA. It voted 8 to 7 against the amendment in 1976. Lobbyists for both sides and Hirst agree that if the committee voted again this year, the result probably would be the same.A full Senate debate on the merits of ERA and a vote on ratification is now scheduled for Thursday, when the resolutioncome up for its third reading. However, there may be some debate on Wednesday when on second reading, amendments to the ratification resolution may be offered. Virginia and Mississippi are the only two states where the ERA has never been debated in either house of the state assembly.

The wording of a proposed U.S. constitutional amendment can not be amended. However, Sen. Frederick T. Gray (D-Chesterfield), a leading strategist among Senate conservatives and an ERA opponent, said in an interview that a proposal to delay the efffective date of Virginia ratification until after a statewide advisory referendum on it could be made on Wednesday.

Today's debate focused exclusively on whether it was proper to take the issue away from the Senate P&E Committee, but with only two exceptions known ERA supporters voted for discharge and know opponents against. One supporter of the amendment, Sen. Hunter B. Andrews (D-Hampton), led the opposition to discharge and an ERA opponent, Sen. James T. Edmunds (D-Lunenberg), favored discharge.

Today's result appears to foretell a narrow majority for the amendment on Thursday. Two senators who oppose it were absent, but their negative votes would only close the margin to 21 to 19.

In other legislative action, the House yesterday passed, 72 to 20, a bill allowing the use of "fuzz busters" in the state.

Fuzz busters are automobilemounted electronic devices that detect the presence of radar. They get their name from their ability to warn motorists to slow down when they are in the vicinity of police officers trying to catch speeders. A state police official had argued against repealing the law that banned the devices. The bill now goes to the State Senate.