A judge who is to decide whether Rep. Daniel J. Flood (D-Pa.) is mentally competent to stand a second criminal trial said yesterday that Flood would be acquitted if he "defended himself with the vigor" he showed during cross-examination this week by a government prosecutor.

U.S. District Court Judge Oliver Gasch's comment came as Flood's defense lawyers continued their effort to persuade Gasch that the 76-year-old congressman's mental capabilities are so deteriorated that he does not understand the charges against him and cannot help with his defense.

Doctors who have examined Flood for both the prosecution and the defense agree that he suffers from organic brain disease, commonly associated with old age. A psychiatrist appointed by the court to examine Flood testified yesterday, however, that while Flood does suffer memory lapses, his recollection could be refreshed enough for him to participate in a trial.

Dr. George Weickhardt told Gasch that "showmanship" was important to the longtime politician and former actor. If Flood has an audience, Weickhardt said, and is free of distraction, his performance improves.

Earlier in the week when Flood was cross-examined by prosecutor Mark H. Tuohey III, he was markedly alert and responsive, compared with his halting manner during long hours of questioning by his defense lawyer, Axel Kleiboemer.

Both Weickhardt and Dr. Eugene C. Stammeyer, also appointed by the court to examine Flood, said yesterday that Flood's memory loss and confusion are genuine.

In a report to the court, Stammeyer, the chief psychologist at St. Elizabeths Hospital, said it was "heart-rending" to watch Flood struggle to answer psychological tests, and that he was "doing his level best."

Flood's first trial on conspiracy, bribery and perjury charges ended in a hung jury a year ago. He is charged with taking more than $50,000 in bribes from businessmen in exchange for his influence with billions of dollars in federal grants as chairman of a House Appropriations subcommittee. A retrial is scheduled for Feb. 25.

In the past year, the once-flamboyant Flood has been hospitalized for a variety of ailments and has undergone gall bladder and eye surgery. He is in the Northern Virginia Doctors Hospital, where he is being treated for ulcers.

The competency hearing is to continue today.