A new report on the murder of 6 million Jews by the Nazis concludes that American Jews never mounted a sustained effort to save European Jews from extinction, in part because American Jews were slow to react and in part because they did not believe the Nazis would murder so many Jews.

"American Jews were excessively prudent in reacting to what the Nazis were doing to European Jews," states an interim report of the American Jewish Commission on the Holocaust, chaired by former Supreme Court Justice Arthur J. Goldberg.

"They were so benumbed by the magnitude of this unprecedented catastrophe--as were the European Jews--that they were slow in moving from knowledge to belief to action," the report said.

The report may be revised as the commission's 35 members examine and study it, said Seymour M. Finger, the commission's research director. He said he expects no major changes in the interim report's overall conclusions.

If all goes according to plan, the commission is to adopt the report late this year or in early 1984, Finger said.

The commission report also concluded that American Jews were poorly organized and poorly financed during World War II to mount the kind of all-out effort that would have been required to have any impact on the Holocaust.

"It should be recognized that the American Jewish community of 1939-45 was not the community as we know it today," the report said. "It had little power or influence, and most Jews were first- or second-generation Americans, still trying to pull themselves up by their bootstraps. In general, they were neither affluent nor influential."

The report is the second interim report of the commission, a privately funded organization based in New York and formed in 1981 at the initiative of several influential and wealthy American Jews. The new report is not as strongly critical of American Jews as was the first report, which cast American Jews as "indifferent" to the plight of the Europeans.

Not only did the first report anger American Jews, it also led the commission's chief financial backer, a wealthy businessman and Holocaust survivor, Jack Eisner, to withdraw his support from the commission.

With impetus provided mainly by Goldberg, the commission pressed on despite Eisner's withdrawal.

Even if American Jews had been more organized and determined to spotlight the plight of European Jews, the report concluded, there was little they could have done to reverse the Holocaust.

"Would it have had a major impact on the rescue of European Jews?" the report asked. "We doubt it. The number that could have been saved once the Holocaust started was extremely limited.

"Nothing we say or do can bring back even one of the 6 million dead, and nothing can make up for their loss," the report said. "Yet at least we should learn from the experience, and we believe there are profound lessons to be derived from it."

Many Holocaust scholars said they felt that the new report was an improvement on the first one, which said American Jews "were afraid to be bold enough" to mount an all-out effort to halt the Holocaust.

"I think this report constitutes a fair and balanced report of the situation," said Hyman Bookbinder, Washington representative of the American Jewish Committee. "On the whole, it sounds like a moderate set of conclusions."