Present and former members of Congress accused of corruption areimmunized by the Constitution from introduction of evidence at a trial of their acts as legislators, the Supreme Court ruled yesterday.

Such immunity is granted by the clause providing that "For any speech or debate" in either the House or Senate, a representative or senator "shall notbe questioned in any other place," the court held.

For a five-member majotity, Chief Justice Warren E. Burger said there was not doubt that exclusion of evidence of legislative acts "will make prosecutions more difficult." The Justice Department had gone further. Arguing that "in a large number on cases" bribery prosecutions of federal legislators would become "virtually impossible."

But Burger said, the clause "was designed to preclude prosecution of members for legislative acts" so as "to preserve the constitutional structure of seperate, co equal and independent branches of government."

Burger wrote in a case in which former representative Henry Helstoski (D-N.J.) was indicted on charges of conspiring to defraud, and having defrauded, the United States by accepting bribes from aliens to allow them to immigrate through introduction of bills in Congress.

Justice William J. Brennan Jr. dissented, saying, "I would go much further and order the dismissal of Helstoski's indictment altogether."

Justice. John Paul Stevens agreed with Burger that past rulings involving two Maryland Democrats, former representative Thomas F. Johnson and former senator Daniel B. Brewster, prevent prosecutors from trying to prove that an accused legislator had performed a legislative act.

Dissenting in part, however, Steven wrote that those rulings don't "require rejection of evidence that merely refers to legislative acts when that evidence is not offered for the purpose of proving the legislative act itself."

Justice Potter Stewart signed Steven's opinion.Justice Lewis F. Powell Jr., who had been ill, abstained.

Although the ruling preserved the indictment, it raised doubts whether the government has a viable case to bring to trial.

Helstoski, contending that the indictment was invalid because the grnad jury had heard evidence of legislative acts in violation of the speech-or-debate clause, had asked U.S. District Court Judge H. Curtis Meanor to dismiss it.

Meanor refused. Helstoski then asked the 3rd U.S. Circuit Court of Appeals for a writ of mandamus - an order to Meanor to dismiss the charges. The appellate court declined, holding the indictment to be valid on its face.

The 3rd Circuit was upheld yesterday, 7 to 1, with Brennan the sole dissenter. Direct appeal to the 3rd Circuit simply for a ruling on Meanor's action was "the proper course," Burger said in the majority's opinion.

In the dissenting opinion, Brennan said that Helstoski "may well be excused if he views the court's holding as if it were a line out of Joseph Heller's "Catch-22." He cannot utilize mandamus because he should have sought a direct appeal. But he cannot seek a direct appeal, because that avenue is time-barred," i.e., it's too late. late.

The indictment developed from a long series of investigations in New Jersey into private immigration bills, which usually go through Congress routinely. Helstoski had introduced 169 such bills.

In 1975, former Helstoski aide Albert DeFalco was indicted and convicted for having fraudulently represented himself as working for the legislator when he took money allegedly for the introduction of immigration bills by the congressman.

The prosecution, led by U.S. Attorney Jonathan L. Goldstein in Newark, stipulated that Helstoski, a defense witness, wasn't in on the conspiracy.

Between April 1974 and May 1976, Helstoski appeared - voluntarily - 10 times before grand juries. Each time he was told he had certain constitutional rights, and each time he apparently assumed he wasn't a target. "I promise full cooperation with your office, with the FBI, this grand jury," he said in what Burger called a "typical" exchange with a prosecutor.

Starting with his fourth appearance, however, Helstoski began raising objections - to the burden imposed by requests for information, and to certain conduct of the U.S. attorney. In his ninth appearance, in May 1976, he asked if he was a target. The prosecutor wouldn't tell him. Helstoski then implied for the first time that he was relying on the speech or debate clause to protect him.

The indictment was returned in June 1976. That was a week before the primary, in which Helstoski sought but failed to win renomination for a seventh term.

In the lower courts, the government contended that legislative acts could be introduced to show Helstoski's motive for taking bribes, and that Helstoski implicitly had waived his immunity under the clause.

Agreeing with the appeals court, Chief Justice Burger wrote that Helstoski's legislative acts - introduction of the immigration bills - can't be admitted at a trial "without undermining the values protected by the clause."

The protection "extends only to an act that has already been performed," Burger emphasized. "A promise to deliver a speech, to vote, or to solicit other votes at some future date is not 'speech or debate,' " he said. "Like-wise, a promise to introduce a bill is not a legislative act."

Burger also agreed with the 3rd Circuit that if a legislator is to waive his immunity under the clause, he must do so with an "explicit and unequivocal renunication . . ."

Justice Department officials said yesterday that the Helstoski decision didn't appear to threaten the pending bribery case against Rep. Daniel J. Flood (D-Pa.) because Fllood is accused of taking money for influencing executive branch agencies, not legislation in Congress.

It is likely, though, that the decision would hurt cases like those the government brought against former representatives Otto E. Passman (D-La.) and Richard T. Hanna (D-Calif.) during the investigation of South Korean influence buying in Congress.

In both those cases, the indictments referred to letters the two men wrote to the President of Korea, outlining actions they had taken in Congress to help Korea or Seoul lobbyist Tongsun Park.

Some prosecutors contacted yesterday noted that they always have been careful to avoid "speech and debate" problems in potential cases against members of Congress.