WASHINGTON (AP) _ The independent counsel in the Iran-Contra affair appealed to the Supreme Court on Tuesday to reinstate the conviction of former National Security Adviser John M. Poindexter.

Independent counsel Lawrence Walsh said a 2-1 appeals court ruling that overturned Poindexter's conviction threatens numerous federal corruption prosecutions and is at odds with rulings by a host of other federal judges.

''Generations of legislators, judges and defendants have had no trouble understanding'' a law barring obstruction of Congress that was struck down by the appeals court in Poindexter's case, Walsh said in a legal brief.

The majority on the appeals court reached ''the remarkable conclusion that reasonable people would not understand the obstruction of Congress statute to reach those who lie to a congressional committee with the purpose of thwarting a congressional inquiry,'' he said.

The Supreme Court began its 1992-93 term on Monday. It could be months before it decides whether to hear Walsh's appeal in the Poindexter case.

Voting 2-1, a three-judge panel of the U.S. Circuit Court of Appeals here last November reversed all five Iran-Contra charges against Poindexter.

The appeals court said the prosecution failed to prove Poindexter's immunized testimony to Congress was not used against him at trial. And the panel said Poindexter's two convictions for obstruction of Congress must be reversed because the law is too vague to prohibit lying to Congress.

Corruptly influencing a congressional inquiry does not clearly encompass lying to Congress, the judges said.

Theoretically, Walsh's office could retry Poindexter on some of the charges if it loses the high court appeal. But the practical obstacles, underscored by the Oliver North case, make that improbable.

The Supreme Court in May 1991 let stand an appeals court ruling that set aside former White House aide North's convictions, and the case against him later was dismissed.

Poindexter, a key figure in the Iran-Contra scandal during the last years of the Reagan administration, was convicted in April 1990 of conspiring to conceal the truth from Congress.

The five felony counts charging him with a cover-up included one of conspiracy, two of obstruction of Congress and two of false statements. He had been sentenced to six months in jail and a $250 fine.

Poindexter, like North, testified at widely publicized congressional hearings in 1987 under a limited grant of immunity.

The two appeals court judges who overturned his conviction were Douglas H. Ginsburg and David B. Sentelle, both appointed to the bench by President Ronald Reagan. Dissenting was Judge Abner J. Mikva, who was appointed by Jimmy Carter.

Walsh is ending his five-and-a-half-year investigation of the Iran-Contra scandal. He plans to prosecute former Defense Secretary Caspar Weinberger and former CIA official Duane Clarridge and to retry former CIA spy chief Clair George, whose first trial ended with a deadlocked jury.

The Iran-Contra affair involved secret arms sales to Iran, with the proceeds going to support Nicaraguan rebels.