Federal prosecutors suddenly have renewed their attempt to find the individual they believe set out to scuttle now-suspended Gov. Marvin Mandel's first trial on political corruption charges, despite the fact that Mandel's conviction is on appeal and one man convicted of jury tampering already has served his time and has gone home.

Walter E. Welkers, the Baltimore furniture salesman found guilty 21 months ago of attempting to bribe a juror in Mandel's first trial, appeared before the grand jury for several hours Tuesday.

Those familiar with the investigation believe that it is also focusing on possible links to Charles Edward Neliswender, the self-described "con-man" from Cinnaminson, N.J., who was convicted of obstruction of justice for a separate attempt to disrupt the 1976 trial of Mandel.

A mistrial was declared by U.S. District Court Judge John Pratt when several jurors overheard television news reports about the tampering attempts.

Also one Tuesday, Benjamin Greenberg, a 60-year-old roofer from Cherry Hill, N.J., appeared briefly before the grand jury.

Then, according to persons familiar with the case, he was taken before U.S. District Court Judge Alexander Harvey, who signed an order sought by the prosecutors of compel Greenberg to testify before the grand jury. Greenberg was granted "use immunity," meaning that his testimony could not be used against him in any possible case.

Greenberg, brought before the grand jury again has been ordered to appear before Harvey a second time to show why he should not be held in contempt of court.

Normally, a grand jury witness is granted "use immunity" only after he has refused to testify, asserting his Fifth Amendment right against self-incrimination.

Those familiar with the current investigation say it was renewed to make one more attempt to find the person, dubbed "the shark" by prosecutors, who they believe was trying to subvert the trial. It appeared that new evidence did not trigger the current investigation.

The 69-year-old Weikers was convicted in February 1977 of obstruction of justice for offering a $10,000 bribe to Mandel juror Oscar Sislen, a relative by marriage. Sislen reported the attempt to the presiding judge in the case and was removed from the jury panel.

Neiswender, whose case is on appeal, was convicted last year of obstruction of justice. Prosecutors charged that he offered to fix the trial for $20,000. He made the offer to an undercover U.S. postal inspector, who was masquerading as a man associated with Mandel's defense lawyer, according to testimony at Neiswender's trial.

Neiswender said he was acting as an intermediary for someone else who was "in politics a little bit" in Maryland, according to testimony at his trial.

Government agents were brought into the case after Mandel's lawyer, Arnold Weiner, told them he had been contacted about the jury-tampering offer.