Federal prosecutors today asked the judge in the political corruption trial of Gov. Marvin Mandel to reject the governor's latest request for a postponement because of poor health and start the trial next Wednesday as scheduled.

The prosecutors cited a letter written to U.S. District Court Judge Robert L. Taylor from one of Mandel's physicians as evidence that "no further postponement . . . is legally justified."

The letter, written by Dr. George B. Udvarhelyi told the judge that Mandel's headaches have improved, that there is no evidence of a speech impairment and that there had been a 50 per cent improvement in his condition. It was dated April 29, two days after Mandel was released from Johns Hopkins University Hospital.

Chief prosecutor Barnet D. Skolnik noted that Udvarhelyi earlier had suggested that the governor have a one-month period of recuperation following his discharge from the hospital. Arnold M. Weiner, the governor's attorney, used that recommendation as the basis for filing a motion last week seeking postponement of the trial until June.

Today's action by the prosecutors noted that the doctor's recommendation was that Mandel should "be away from the stresses of his usual occupation and should be away from his usual environment to allow further recovery." Skolnik wrote that if the trial proceeds as scheduled on May 11, the process of selecting a jury would likely last until nearly the end of the one-month recuperation.

The prosecutor told the judge that during jury selection, defendants are "essentially observers" and it is not necessary under the law for Mandel to even attend that phase of the trial.

"Should he choose to attend, it cannot reasonably be claimed that his passive role during jury selection is likely to generate a situation of great stress," the prosecutors said.

Skolnik said further that even once the presentation of evidence begins Mandel "will be spending several weeks merely observing the presentation of the government's evidence, the vast bulk of which he has seen before, at his first trial."

The first trial was aborted last Dec. 7 as the result of jury tampering. Most of the 13 weeks of testimony was devoted to the prosecutor's side. The defense had just begun to present its version of the story when the trial ended abruptly.

The prosecutors said that "at the point of which he (Mandel) will be called upon to present his defense, he will have had a recovery period far in excess of the doctor's one-month recommendation."

Weiner, Mandel's lawyer, strenuously disputed this. "One must abandon human experience to suggest . . . that a defendant on trial for the most serious charges, and whose entire future hangs in the balance, view the proceedings passively, and with no more emotional detachment than a spectator at a ball game," he said in a formal reply to the prosecution motion.

He said that Mandel, a lawyer himself, played a crucial role in selecting the jury in his first trial, taking notes and suggesting questions to ask prospective jurors. It is unreasonable to suggest Mandel "should abandon his right to do so again in his second trial in order to preserve his health and life," he said.

Weiner also noted that the government made its motion without hearing the conclusions of the neurologist hired by the prosecution to examine Mandel, implying prosecutors may be afraid to learn his conclusions.

The prosecutors went on to say that even if brief interruptions or adjustments to the trial schedule were needed occasionally, the judge could make appropriate arrangements "thereby allowing this case to be tried to a conclusion once and for all."

The letter from Dr. Udvarhelyi, a professor of neurosurgery at Johns Hopkins, said that a repeated examination of tests first conducted on Mandel at Prince George's General Hospital "still revealed some motor weakness on the right side, mininal in coordination of the right leg and creet sensory disturbance of a cortical type." The letter then added the phrases that the prosecutor cited about the improvement in the governor's condition.

Udvarhelyi said a study of the result of brain test performed April 25 "was interpreted as normal, not showing any evidence at the present time of a tumor, vascular abnormality or any other detectable disease of the blood vessels."