A federal jury today convicted 52-year-old Charles E. Neiswender of obstruction of justice, making him the second man to be found guilty of tampering with last fall's aborted political corruption trial of Maryland Gov. Marvin Mandel.

However, none of the evidence produced in Neiswender's case, and nothing said in the five-day trial of Walter Weikers earlier this month gave the government any new clue to the identity of the people who, prosecutors believe, instigated the tampering efforts.

According to sources, neither Neiswender nor Weikers has given prosecutors any useful information to lead them to the person or persons they figuratively call "the shark," who, prosecutors believe, set into motion the partially successful tampering efforts.

A mistrial was declared in the case of Mandel and four codefendants last Dec. 7, after several jurors inadvertently overhead a news bulletin about the month-old secret of Neiswender's arrest.

The 52-year-old self-described "conman" from Cinnaminson in southern New Jersey was convicted today of soliciting a $20,000 bribe from Mandel's chief defense attorney to give to a juror who, Neiswender claimed, was willing to throw the case for a price.

However, in a sworn statement given to the Mandel trial prosecutors shortly after his arrest in November, Neiswender said that he had been paid $5,000 "to throw a snag in the trial."

The idea, Neiswender told prosecutors then, was to solicit a bribe from Arnold Weiner, Mandel's chief defense counsel, knowing that the attorney would immediately report the attempt to the court and prosecutors.

The resulting investigation and publicity, Neiswender said in his statement, would probably lead to a mistrial in the case - exactly what happened less than a month after Neiswender first reported this supposed plan.

However, even though Weikers and Neiswender each face penalties of up to five years in prison and $5,000 fines for their obstruction of justice convictions, the two men's silence or lack of knowledge about the people behind the tampering effort has left prosecutors with a huge barricade between them and the "shark."

Authorities are said to have expected greater cooperation from Weikers after his conviction Feb. 4 of offering a $10,000 bribe to a Mandel juror to hold out for acquittal.

And, while Neiswender with a spotty record of convictions and an uneven employment history, seems to fit the role of a know-nothing go-between, authoritiese are baffled by the spectacle of the 67-year-old Weikers, an ailing man obviously proud of his clean record, stonewalling while facing the possiblility of ending his life in prison.

So far, however, these expectations have not been fulfilled. Three weeks after his conviction, the 67-year-old suburban Baltimore furniture salesman has been no more help than he was before the jury came back, sources have said.

Neiswender, who at one point told The Post that he was cooperating with the prosecution, may have nothing useful to tell them. In one early statement, to prosecutors, he claimed he was originally approached by a man he didn't even know.

"The type of guys who do this dirty work in the first place are good at" insulating themselves from authorities and preventing their hirelings from talking, said one source familiar with the probe.

Said another, "This guy [the shark] is a lot more scared of his friends right now than he is of any prosecutors." The source added, however, that the "shark" may be well insulated not only by layers of people but by a substantial amount of money.

During the final arguments in Neiswender's case yesterday, defense attorney J. Frederick Motz, however, completely avoided the idea that any shadowy figure Neiswender's attempt, painting his client instead as a lone and bungling con man.

While admitting the validity of much of prosecution evidence - including an hour and a half of tape recordings of Neiswender discussing the deal with a federal agent posing as be Weiner's law partner - Motz argued that his client only intended to con Weiner, not to obstruct justice.

"Mr. Neiswender was engaged in an old-fashioned confidence game," Motz said. "He was a con artist - and not a very good one."

After, reciting a litany of Neiswender's bungles - his charging a plane ticket in his own name during a period in which he was using the alias "Lee Anderson," his furtive conversations from pay phones, which were continually interrupted by operators demanding more money - Motz asked, "can one perceive that these were the actions of a mab intending to obstruct justice?"

In his closing argument, however, Asst. U.S. Atty. Ronald Liebman anticipated this argument, telling the six men and six women on the jury, "When you say to a lawyer, who has a duty to perform, something that's designed to affect the performance of that lawyer's duty, you have obstructed justice."

An attornery who has paid a bribe, Liebman said, "isn't going to work the same way. Would you? Because it'll be in the back of your mind, 'It's in the bag.'"

Motz's opening and closing arguments in the trial constituted the whole of the defense case Neiswender did not take the stand and, because of a prior agreement, his sworn statement was not introduced as evidence.

At the close of the prosecution's case on Thursday, Motz and his associate Charles M. Kerr asked the judge to acquit their client, saying that the federal law and the legal precedents showed their client's actions did not constitute obstruction of justice.

It took the jurrors only slightly more than two hours to reach their verdict.

Judge Frank Kaufman then refused a request by Liebman to lift Neiswender's personal recognizance bond and take him into custody - but the judge confined Neiswender to the immediate area of his home and his local unemployment office pending sentencing.