Former Maryland governor Marvin Mandel's political corruption conviction was reinstated yesterday by a 3-to-3 vote of a federal appeals court in Richmond.

Mandel's attorney said immediately he would appeal the decision, perhaps the final stage in a legal saga that began in 1975. It carried the former governor through one mistrial, one conviction, one reversal and culminated in a dramatic 451/2-hour return to the governor's chair last January.

With yesterday's decision, Mandel once again faces a four-year jail term, but when his lawyer called him with the decision at 11:45 a.m. yesterday, he responded by asking for more legal details and then disappearing from the area.

In a two-sentence order, the 4th U.S. Circuit Court of Appeals reversed the decision reached last January by a panel of its own members. That panel had overturned Mandel's conviction, ruling that the judge who presided at his 1977 trial committed several serious errors.

Government prosecutors sought and won a rare rehearing before the entire court last June 5.

The full court's tie vote automatically reinstated the Jury's 1977 verdict that found Mandel and his five codefendants guilty of mail fraud and racketeering. One member of the court, Harrison L. Winter of Baltimore, disqualified himself from hearing the case.

Under appellate procedures, a tie vote upholds the action of the lower court being reviewed.

Mandel was convicted in August 1977 of 17 counts of mail fraud and one count of racketeering after prosecutors charged that he had accepted $350,000 in gifts, vacations and stock from wealthy friends in return for secretly using his office to enrich their business interests.

These friends -- Irvin Kovens, Harry W. Rodgers, William A. Rodgers, W. Dale Hess and Ernest N. Cory -- were Mandel's codefendants.

The convictions of all six men had ended Mandel's second trial -- the first had ended in a mistrial eight months earlier -- and made him first Maryland governor in this century to leave office in disgrace. The events leading to that moment had begun in November 1975 when Mandel and the others were indicted by a grand jury after an investigation of the U.S. Attorney's Office in Maryland.

Yesterday, nearly four years after the indictment, Mandel's attorney, Arnold M. Weiner, plotted strategy for the next round in his Baltimore office.

"This case is not over yet," he asserted.

Weiner said he has three major options now. He could appeal to the same six judges who ruled yesterday, asking them to make a "further effort at deciding the issues." He could also ask the court to hear the case all over again -- this time with its two newest judges joining in. Both were appointed and confirmed after the appeal was argued in Richmond June 5.

Weiner said he could also decide to seek a review by the U.S. Supreme Court. But he would have to convince the justices that the issues were of such magnitude that they should consider the case.

One of those issues, according to defendant Hess' attorney William Hundley, is the 4th Circuit's tie vote.

"i certainly wouldn't want to go to jail on a tie," Hundley said. "In tennis there's a tie-breaker. In baseball there are extra innings. Shouldn't you at least have the same right in a criminal appeal?"

The appellate tug-of-war so far has focused on three issues.

The defense has contended that Judge Robert L. Taylor, who presided at Mandel's 1977 trial, allowed the prosecution to use Improper "hearsay" testimony by several senators to prove that Mandel wanted his veto overridden on a piece of race track legislation.

The race track deal was a key to the prosecution's case, and its primary evidence of the deal was an intricate legislative maneuver it accused Mandel of masterminding.

1when the run-down Marlboro Race Course in Prince George's County was owned by some Western Maryland businessmen, Mandel vetoed a bill that would have doubled the number of lucrative racing days allocated to the track.

After the veto, Hess, Rodgers, Kovens and Cory secretly purchased the track at a price drastically reduced because of the governor's action.

In 1973, in the next lelgislative session when lawmakers routinely consider overriding vetoes, the prosecutors charged that Mandel quietly worked to have his own veto overturned. The track suddenly took on new valve.

The defense argued that the judge had improperly allowed state senators testify about the alleged override maneuver by citing comments made on the Senate floor, from which the senators said they inferred that Mandel wanted the vote overridden.

The defense, in its appeal to the 4th Circuit, also argued that Judge Taylor allowed the jury to consider other improper evidence and failed to instruct the jurors on the relevant law in the case before they retired to deliberate.

It was precisely on these issues and the "hearsay" question that the appellate panel last January based its reversal of the convictions. But, according to yesterday's terse decision, three judges on the full court rejected all these claims by the defendants.

Weiner said because of that 3-to-3 split he would add one new issue in his next appeal. He will argue that a tie vote cannot uphold a criminal conviction because an even split means the judges failed to reach any conclusion at all.

Assistant U.S. Attorney Daniel Hurdson, one of three prosecutors who tried the Mandel case, refused to speculate on any further appeals, saying only: "We're very pleased with the 4th Circuit's decison."

But noted criminal appellate expert Charles Alan Wright, of the University of Texas, said he was "not impressed by" Weiner's argument. Wright said that en banc (full) courts previously have upheld civil decisions by tie voters, and "I don't see why there should be a special rule for criminal cases."

The 4th Circuit's brief ruling affirming the convictions left much in the complex case open to speculation. The only opinion in the case came from the two judges -- H. Emory Widener Jr. and Donald S. Russell -- who strongly dissented from the court's action.

The reasoning of the other four judges -- Chief Judge Clement Haynsworth, John Butzner, Kenneth Hall and Dickson Phillips -- remained a secret.

One of the four joined the disenters on one issue, finding merit in the defense claim that the trial judge had given inadequate jury instructions.

Widener, one of the two dissenters, wrote that this inadequacy -- the judge's failure to tell jurors the legal definition of a bribe when explaining mail fraud to them -- was serious enough to warrant reversing the convictions.

Widener took his fellow judges to task for failing to write an opinion in the case, particularly on this point. He stated that affirming the convictions despite this error "is so far from settled precedent that the reasoning should be stated in the opinion of the court."

Widener also found that the "hearsay" testimony of the state senators at Mandel's trial was the kind of "gossip rumor, unfounded report, second, third or futher-hand stories," that previous appellate rulings and decried.

The reinstatement of the convictions means that the defendants could be ordered to start serving their sentences, pending further appeals. But government prosecutors, who would have to ask the court to orders this, have not yet decided if they will.

The defense attorneys have 14 days to file a petition for rehearing before the 4th Circuit, and 30 days to petition the Supreme Court for review, according to Weiner. CAPTION: MARVIN MANDEL . . . faces jail term