The legal tug-of-war over the reversal of former Maryland governor Marvin Mandel's political corruption conviction was renewed here today as prosecutors and defense attorneys debated whether crucial hearsay testimony should have been allowed into evidence in the trial.

The arguments came at an unusual rehearing of the case before the full 4th U.S. Circuit Court of Appeals here, five months after a three-judge panel of the same court voted to overturn the convictions of Mandel and his five codefendants.

The original appellate decision cited a number of procedural errors made by the presiding judge at Mandel's 1977 trial, and the bulk of the arguments today centered on one of those questions.

In the split, 2-to-1 decision of the panel, Judges H. Emory Widener and Donald S. Russell ruled that the trial judge had allowed the prosecution to use improper "hearsay" testimony by several state senators to prove that Mandel wanted his veto of a piece of racetrack legislation overridden.

The prosecution contended that Mandel switched positions on the legislation - and allowed the Maryland Senate to override his veto - in return for financial favors from his codefendants, who owned the Marlboro Race Course. The overriding of the veto enhanced the value of the race course.

Yesterday, Mandel's attorney, Arnold Weiner, argued that the "hearsay testimony" of the senators, based on "rumors and gossip" on the Senate floor, was the only evidence the prosecution had to prove that Mandel "brought about the veto of his own legislation.

But Judge Kenneth D. Hall asked Weiner, "Why wasn't the judge correct in permitting the evidence in, not to prove the truth of the rumors, but to prove the rumors existed?"

Weiner answered that the jury was allowed to "decide the case on gossip, rumor, and hearsay," and "a criminal case should not be permitted to go to the jury on that kind of evidence."

But Assistant U.S. Attorney Daniel Hurson argued that the "hearsay issue has become magnified," but that it was only one part of a major case proving mail fraud against the former governor.

Judge Widener retorted, ". . . what evidence was there, other than hearsay, on the veto override?"

Chief Judge Clement Haynsworth also wanted to know about the prosecution's "proof" that Mandel had influenced legislation to benefit his codefendants.

Hurson answered that there was "an abundance of circumstantial evidence that the governor didn't care if his veto was overriden," including testimony that Mandel's lobbyists had "backed off and did nothing to sustain the veto."

During the course of Hurson's arguments, he was subjected to the same tough grilling from Judge Widener that he had faced during the original appellate hearing.

As the prosecutor summed up his case, telling the judges, "This was a long and important prosecution - perhaps the most significant in the state's history," Judge Widener cut him off.

"Are you saying it's such a big deal that we shouldn't reverse it?" the judge asked.

The court's final ruling in the case will depend on the votes of the three appeals judges - Hall, Haynsworth and Dickson Phillips - who did not sit on the original panel, legal observers said. It is considered unlikely that any of the three original panel judges would change their minds.

Justice Department officials have said that a 3-to-3 vote in the case would affirm the convictions.

Mandel and his codefendants - Irvin Kovens, Harry Rodgers, William A. Rodgers, W. Dale Hess and Ernest N. Cory - were convicted in August 1977 on mail fraud and racketeering charges. The prosecution had accused the former governor of accepting about $350,000 in gifts, vacations and stock from these wealthy friends in return for using his office to enrich their business interests.

The appellate judges today also questioned the lawyers about the trial judge's decision to allow jurors to consider the Maryland Code of Ethics during their deliberations. The panel had ruled that because the code did not apply to the governor, this was improper.

Judge Phillips asked defense attorney Weiner whether the code wasn't "just an obvious homily" about what the public can expect from its elected officials.

Weiner agreed that it was and then said that the prosecution's case had been based on just such a "homily," instead of an actual proof of wrongdoing by the former governor.

Weiner also argued that the jury should have been instructed that it must find Mandel guilty of accpeting a bribe from his friends in order to convict him on mail fraud charges.

The panel had ruled that presiding trail Judge Robert L. Taylor had erred by failing to instruct the jury on this point.

However, Judge D. Butzner had dissented strongly not only from this decision, but also from other key points in the majority ruling.

It is not known when the six appellate judges will rule in the case.

Judge Harrison L Winter of Baltimore, also a member of the 4th Circuit, has disqualified himself from considering the Mandel case. CAPTION: Picture, MARVIN MANDEL . . . veto of racetrack bill cited