WASHINGTON (AP) _ Lyn Nofziger's illegal lobbying convictions should be overturned because prosecutors didn't prove the former White House aide willfully violated the ethics law, his lawyer told an appeals court Tuesday.

The Ethics in Government Act of 1978 only vaguely defines criminal intent and illegal lobbying contacts so that is it impossible for a person to know if he is violating the law, attorney Andrew L. Frey told a panel of the U.S. Circuit Court of Appeals here.

Frey contended that the trial judge should have required independent counsel James C. McKay to prove that Nofziger knew the issues on which he was lobbying were of ''direct and substantial'' interest to the White House.

Because there was no proof that Nofziger had this specific knowledge, he can't be held criminally liable for trying to interest the White House in issue that concerned his lobbying clients, Frey argued.

''The concept of direct and substantial interest as espoused by the independent counsel and the court is so incapable of precise and predictable application as to make the statute unconstitutionally vague,'' Frey argued.

Nofziger was convicted earlier this year of three felony counts of violating the one-year ban on lobbying his former agency by contacting former White House colleagues within a year of resigning as President Reagan's political director in early 1982.

He received a 90-day sentence and was fined $30,000 by U.S. District Judge Thomas A. Flannery, who allowed Nofziger to remain free while the case was appealed.

Nofziger, the first former high government official to be convicted under the law, is being supported in his appeal by the American Civil Liberties Union, which filed a friend-of-the-court brief.

He is challenging Flannery's interpretation of the legal requirement for proving criminal intent under the law.

Flannery ruled the prosecution need only prove that Nofziger ''knowingly'' represented clients at the White House within the one-year period in which his lobbying was restricted.

The judge held that proof that Nofziger intended to influence the White House was also sufficient to convict the former presidential aide of two counts of illegally sending letters to former administration colleagues.

That element was not needed to prove criminal intent in a third count charging Nofziger attended a White House meeting to urge continued funding of an antitank plane, the judge ruled.

But ''the jury was not asked to find that Mr. Nofziger had knowledge of the circumstances that made his contacts with the White House unlawful,'' Frey argued.

Frey maintained because the statute is so confusing, ''all we have to do to prevail here is persuade your honors that there is some ambiguity in the law.''

Richard Friedman, an associate independent counsel, argued that the prosecution presented sufficient evidence that Nofziger intended to exert influence when he wrote then-White House counselor Edwin Meese III on behalf of Wedtech Corp.'s bid for an Army contract, and Meese's deputy, James Jenkins, on behalf of a maritime engineers union.

Friedman also argued that the statute clearly states the prosecutors only had to prove that Nofziger knowingly acted on behalf of Fairchild Republic Corp. when he attended a Sept. 24, 1982 meeting with White House staff members on continued appropriations for the A-10 antitank plane.

''The intent of Congress is stated with exceptional clarity in the conference report'' written before the law was enacted, Friedman said.

But two of the panel members, Circuit Judges James L. Buckley and Stephen Williams, appeared troubled by this argument.

Buckley asked Friedman to provide more evidence that Congress intended a broad rather than a narrow definition of criminal intent, but the prosecutor said he could cite nothing else.

Friedman also argued that if there was any doubt in a former official's mind about whether a lobbying conduct was lawful, he could seek a ruling from the Office of Government Ethics.

''Would you recommend a client of yours to go forward on the basis of that recommendation,'' Buckley asked.

''I would recommend my client stay clear. I don't interpret the law to create a broad safe harbor of lobbying by former officials,'' Friedman said.

''It sounds like by your standards that the only safe place to go is out of Washington for a year,'' said Circuit Judge Harry Edwards.