The likelihood that the Virginia General Assembly will pass the Equal Rights Amendment remained slight today as House Speaker A.L. Philpott gave ERA opponents a majority on a critical legislative committee.

Philpott, an opponent of the amendment, appointed four anti-ERA delegates to six vacancies on the House Privileges and Elections Committee, where the measure has died in previous years. The appointments, which cannot be challenged under the legislature's rules, make it almost certain that ERA supporters will have to attempt to bypass the committee if the amendment is to reach the House floor.

Prospects for such a tactic, considered the last chance for the ERA in Virginia, are poor. An ERA opponent last week made the unusual move of introducing a ratification bill on his own, allowing it to be dispatched to the elections committee before ERA supporters could act.

Gov. Charles S. Robb today gave the embattled supporters their only encouraging news of the day, calling for ratification in a message to a joint session of the House and Senate. Del. Dorothy McDiarmid (D-Fairfax), leader of the ERA forces, said she was "very pleased" by Robb's statement.

"I think no governor has ever said without any equivocation, 'I am for this, it is right,' " said McDiarmid, "He was very helpful."

The amendment, which bans discrimination on the basis of sex, has been approved by 35 of the needed 38 state legislatures. It is scheduled to die this summer unless three more states approve ratification. The Virginia legislature has failed to approve it for the past nine years.

Philpott's committee assignments took few legislators by surprise. "It's anti-ERA, I can tell you that," said Del. Warren Stambaugh (D-Arlington), one of the staunch ERA supporters in the Northern Virginia delegation.

"I was at a dinner for Young Democrats two years ago when Philpott said he would stack the committee against ERA ," said Marianne Fowler, state coordinator of Virginians for the ERA, "I think it's a comment on his integrity and on the power he holds that he did it."

Del. Marian Van Landingham, a freshman Democrat from Alexandria, was one of two ERA supporters named by Philpott to the elections committee, an unusual honor for a first-term delegate. The other ERA proponent put on the committee was Del. James Dillard (R-Fairfax), who was serving on the committee when he was defeated for reelection in 1977. Dillard was reelected in 1979.

Some delegates are angry over what Del. Mary Marshall (D-Arlington) called an attempt by Del. Lacy Putney (I-Bedford) to do "an end-run around Dorothy."

McDiarmid said she too was "furious" when she learned that Putney, the House's only Independent member, introduced the ratification measure.

Putney said today he didn't intend any disrespect. "I think the world of Dorothy McDiarmid. She's a tremendously fine lady. I was really interested in getting the issue over with," said Putney, the 53-year-old lawyer who fears that the ERA might force the famed Virginia Military Institute, located in his district, to accept women as students.

Both sides in the debate are researching parliamentary rules to see whether Putney's action would block ERA supporters from asking the House to constitute itself as a committee in order to consider the bill on the floor. "There is some serious question about whether that can be done since the matter is already physically residing in a committee," said Del. Claude Anderson (D-Buckingham), the new chairman of the elections committee.

ERA supporters also can ask the full House to discharge the committee of its responsibility to review the legislation. This approach is fraught with problems since it would ask delegates to challenge the time-honored committee system. "It goes to the core of the legislative process," said Anderson, "No committee much likes to be discharged."

Philpott's committee assignments today also angered Republicans, who now hold 33 of 100 House seats. They said the assignments give them unequal representation on the Courts of Justice and Appropriations committees.

Philpott replaced Del. Warren Barry (R-Fairfax) on the Appropriations Committee with Del. Robert Harris (R-Fairfax) after Barry asked for a new assignment. Barry, who owns a property management firm, said the Appropriations work --which often extends through the weekend--was too much of a drain on his business. Another Fairfax Republican, John H. Rust Jr, now in his second term, won a spot on the Courts of Justice committee.