New Virginia court rulings and attorney general's opinions could seriously weaken penalties courts can impose on motorists with multiple convictions for drunk driving in the opinion of some prosecutors, who see them as possibly a major setback to recent efforts to toughen sentencing.

The decisions bar courts and state officials in Virginia from citing drunk-driving convictions imposed on motorists in neighboring North Carolina in determining penalties when the same motorists are convicted of drunk driving in Virginia.

Some Northern Virginia prosecutors say they fear the same decisions could be used to prevent them from citing drunk-driving convictions from the District of Columbia and Maryland in proceedings against repeat offenders in Virginia.

"It would allow them to get off on a technicality," said Arlington Commonwealth's Attorney Henry E. Hudson.

In recent years, lawmakers in Virginia, Maryland and Washington have imposed increasingly stiffer penalties on motorists who are repeatedly convicted of driving while intoxicated. These "habitual offenders" have been the primary targets of Mothers Against Drunk Drivers and other grass-roots organizations lobbying for tougher penalties for convicted drunk drivers.

Hudson said he had asked Virginia legislators, who convene for the 1985 General Assembly session Jan. 9, to change state law to make it legal to use the convictions from North Carolina, and potentially other jurisdictions, in prosecuting drunk drivers.

The Virginia Supreme Court has ruled that the North Carolina law differs too significantly from Virginia law to use convictions against a motorist charged under Virginia's less stringent laws.

Attorney General Gerald Baliles issued an opinion Thursday that the Virginia Department of Motor Vehicles cannot revoke the license of a Virginia driver who is convicted in North Carolina of drunk driving. That opinion paralleled an earlier attorney general's opinion and another state Supreme Court decision that prohibits Virginia courts from considering a North Carolina drunk-driving conviction in sentencing a motorist who was later convicted of driving while intoxicated in Virginia.

Alexandria Commonwealth's Attorney John E. Kloch said he believes the rulings and opinions will have no immediate impact on Northern Virginia cases involving motorists convicted in Washington and Maryland, but said the decisions are "very detrimental" to the efforts of prosecutors in counties that border North Carolina.

But Hudson argued that Northern Virginia jurisdictions could easily be challenged on their use of Washington and Maryland convictions as a result of the state Supreme Court's ruling in the case of Shinault v. the Commonwealth of Virginia and the subsequent supporting opinions of the attorney general. In the Shinault case, the court ruled that a Chesapeake County motorist's conviction of driving while intoxicated in North Carolina could not be used to convict him as an habitual offender after he was found guilty of another drunk-driving offense in Virginia.

Under North Carolina law, a motorist can be convicted of driving while intoxicated with a blood-alcohol level of .10. In Virginia, the standard for drunk driving is a blood-alcohol level of .15.