The full Fourth U.S. Circuit Court of Appeals agreed yesterday to reconsider a decision overturning the mail fraud and racketeering convictions of former Maryland governor Marvin Mandel.

The decision, announced in a brief statement released by the clerk's office in Richmond, opens the possibility that Mandel's August 1977 conviction could be reinstated sometime after the rehearing in June.

Yesterday's order stated that a "majority" of the six judges considering the rehearing request voted to grant it, but the order did not reveal the actual vote.

In a 2-to-1 ruling in January, a three-judge panel of the Fourth Circuit overturned the convictions of Mandel and his five codefendants, ruling that their trial had been marred by a series of technical errors made by the presiding judge in the case.

The U.S. attorney for Maryland, Russell T. Baker Jr., said yesterday that he was "very pleased" by the court's decision. Mandel's attorney, Arnold M. Weiner, said, "Apparently, the full court wants to take a close look at the case."

The court, in its brief order yesterday, gave no reasons for its decision to grant the en banc (full court) rehearing. Such rehearings, according to legal observers, rarely are granted.

Lawyers familiar with the appellate process said the decision could have a wide range of meanings. "It could mean that the judges, because of the importance of the case, just want to give it the full treatment," one knowledgeable lawyer said. "Or it could mean that some of the judges disagree with the findings of the split panel that reversed the convictions."

Six of the court's seven judges will sit as the full court to hear the appeal because Judge Harrison L. Winter of Baltimore has disqualified himself from considering the Mandel case.

Justice Department officials said yesterday that a 3-to-3 vote in the case would affirm the convictions of Mandel and his codefendants.

Mandel was convicted of mail fraud and racketeering after prosecutors charged that he had accepted $350,000 in gifts, vacations and stock from wealthy friends in return for using his office to enrich their business interests.

But Fourth Circuit Judges H. Emory Widener Jr. and Donald S. Russell ruled last January that crucial testimony used by prosecutors to show that Mandel had influenced legislative decisions aiding his friends' business interests never should have been admitted into evidence. The judges also found fault with several other procedural rulings by the Mandel trial judge, Robert Taylor of Knoxville, Tenn.

Federal prosecutors sought the rehearing last February, declaring that the earlier decision marked "a radical departure from prior precedent."

Prosecutors, in their petition for rehearing, agreed with the strong dissent to the panel's ruling written by Judge John D. Butzner Jr., who stated that the trial judge "responsibly discharged his duty."

Neither Mandel nor his codefendants could be reached for comment yesterday. CAPTION: Picture 1,RUSSELL T. BAKER JR.; Picture 2, ARNOLD M. WEINER, Legal opponents in the Mandel case are prosecutor and defense attorney. The Washington Post