Attorneys for Rep. Frank Thompson Jr. (D-N.J.) yesterday attacked the government's indictment of the veteran congressman in the Abscam bribery case, saying the charges should be dismissed because of pre-indictment publicity and the fictions nature of the alleged crimes.

In motions filed in U.S. District Court in Brooklyn, attorneys Daniel A. Rezneck and Stephen E. Kaufman joined the growing number of defense lawyers in the Abscam cases who have cited "leaks" about the investigation as reason for dismissing indictments.

Thompson was charged along with Rep. John M. Murphy (D-N.Y.) and two other men on June 18 with taking cash in return for the promise of legislative favors to FBI agents posing as representatives of Arab businessmen.

Three other House members, Michael O. Myers and Raymond F. Lederer, both Pennsylvania Democrats, and John Jenrette (D-S.C.), have been indicted in the case, the most sweeping congressional scandal in memory.

Thompson's motion yesterday said "leak" is "far to mild a word to describe what has ocurred in this case; there has been nothing like it in the history of the federal judicial system." It quoted from numerous news stories last February that it claimed were false.

For instance, the motion said that, contrary to some reports, Thompson did not struggle with an associate over a briefcase containing $50,000, and did not assure anyone that the supposed sheik, part of the FBI scheme, would never be deported.

Leaks about the investigation have been the subject of an intense internal inquiry by the Justice Department. In Philadelphia on Monday, a federal judge hearing related charges against three city councilmen ordered the department to give him a summary of the results of that inquiry next Monday.

Prejudicial publicity usually has not been deemed serious enough to prevent a fair trial, even in the Watergate cases involving high-level White House officials. But that is because no one has been able to prove that the leaks came from the government. In the Abscam cases, defense attorneys seem to be depending on the government investigation of leaks to prove their point about prejudicial publicity.

Thompson's attorneys attacked the government case on another front, too, saying there could been no criminal violation because the transactions were "imaginary."

"Although self-styled 'representives' of a mythical 'sheik' may have purported to offer money to influence official acts . . . those official acts did not and could never occur," the brief said. "Whatever the Abscam scriptwriters' intent or the efforts of the performers, the play, as it unfolded, involved no criminal enterprise."

The motion said the Justice Department sought to subvert the "speech and debate" protections of the Constitution by "contriving a situation that would involve only fictitious legislative acts." A recent Supreme Court decision has made it more difficult for prosecutors to introduce actual legislation as evidence to prove bribery.

One federal prosecutor said yesterday that there is little case law in the area of fictitious crimes. "We just haven't had many undercover bribery investigations," he said.