An effort by Virginia legislative leaders to enable the General Assembly to override the governor's vetoes received a severe setback yesterday from the State Supreme Court.

Unless the governor vetoes a piece of legislation early in the assembly's annual winter season - something that is rarely done since the governor's practice is to consider signing or vetoing bills after the session ends - there is now no way for the assembly to consider a veto override.

The court said a proposed amendment to the Virginia Constitution allowing the legislature to reconvene for a veto override session cannot be on the ballot in November because it was enacted improperly.

Although the proposed referendum was invalidated on technical grounds rather the substance of the amendment, the question will not come before the voters for a minimum of two years.

The move to permit a 3-to-10-day extra session to allow the Democrat-dominated General Assembly to muster a two-thirds majority to override a gubernatorial veto of legislation has been led by Democrats. However, yesterday's decision was not seen as politically motivated by the measure's leading sponsor.

"I can't say I agree with the court's reasoning," said Senate Majority Leader Adelard L. Brault (D-Fairfax). "But I have no quarrel with the action of the attorney general" in raising the question of the proposal's validity. "He talked to me before bringing it up, and assigned one of his most competent assistants to try the case."

The court decision was prompted by a suit designed to test the validity of the proposed referendum. Acting state comptroller Vincent J. Pross withheld money to print referendum ballots, forcing the attorney general's office to ask the court to require him to release the funds. Thus it was the attorney general's office that argued for including the referendum on the ballot even though it was Republican Attorney General J. Mashall Coleman who raised questions about its validity in the first place.

The court ruled that the Consitution requires that any resolution proposing a constitutional change be approved by two sessions of the General Assembly, the second one being held after new members take their seats after a general election. The problem arose because the resolution agredd to by both houses in 1977 inadvertently included language that would have dropped the rule that the effective date of an appropriations bill opposed to the four-fifths vote needed to alter the normal effective date of a regular bill. Most bills are effective on the "first day of the third month" after a session is adjourned.

In trying to eliminate this loophole, the assembly in 1978 agreed to a version of the resolution that eliminated the section having to do with effective dates. But, the court found, in doing so the 1978 session made it a new proposal.

Brault said that most of the Republicans in the General Assembly are in favor of having the veto override session. Former Republican governor Mills E. Godwin said it wasn't necessary when it was first proposed in 1977: current Republican Gov. John N. Dalton is not opposed to it although his attitude toward it was described by Brault as "lukewarm" because Dalton feeld an extra session would be costly.