LEONARD GARMENT knew that Iran-contra prosecutor Lawrence Walsh would have trouble making any convictions stick.Mr. Garment told Mr. Walsh as much more than four years ago - long before a federal court of appeals overturned the convictions of Oliver North and, this month, John Poindexter. In both cases the appellate court ruled that evidence presented at the defendants' trials had been "tainted" by the men's previous, immunized testimony to Congress on the Iran-contra affair. At the time Garment, a Washington lawyer, represented Robert (Bud) McFarlane, formerly Ronald Reagan's national security adviser. Mr. McFarlane, like Admiral Poindexter and Lieutenant Colonel North, was charged with crimes relating to the secret sale of arms to Iran and the diversion of profits to the Nicaraguan resistance fighters. On behalf of his client, Garment met with independent counsel Walsh in 1987 to discuss the prosecution. Garment recalls: "In my first meeting with Judge Walsh I said, 'It's not your fault, but you're not going to get anywhere with this thing. It's going to be all peanuts, because the evidence is tainted to the hilt. While Suzanne Garment has been analyzing the "politics of scandal" from her perch as a scholar (See book review, Page 13), her husband has viewed it in the dust of the arena. An expert in what he calls "political litigation," Leonard Garment has defended public officials in some of the same prominent Washington investigations his wife dissects. His clients in these cases have included - besides McFarlane - President Nixon during Watergate and former Attorney General Edwin Meese 3d. The most successful practitioners in this form of legal combat unique to Washington, Garment says, are lawyers who understand clearly that the battles are as much about politics, the media, and public perception, as about law. When the client is in public life, a criminal indictment can be as career-shattering as any conviction. Even negative publicity about the client that is not effectively rebutted can usher him out of Washington on a rail, tarred and feathered. A lot of non-Washington lawyers - including some famous criminal-defense attorneys - never fully grasp the political exigencies of cases like Watergate or Iran-contra, Garment says. "A very good criminal lawyer from New York or Boston will come to Washington and will give advice based upon his experience in his home venue, but it will be off the mark," he says. After more than 20 years as a trial lawyer in New York - interrupted by five years as an adviser in the Nixon White House - Garment moved to Washington in 1981, where he joined a law school classmate's firm. A friend put him in touch with Charles Wick, then the head of the United States Information Agency. Mr. Wick had been caught taping his telephone conversations without notifying the other parties. Though it is not illegal in Washington, many people regard the practice as unethical, and critics were c alling for Wick to resign. LEADING the outcry against Wick was Garment's former White House colleague William Safire, a New York Times columnist. Garment speculates that Wick hoped Garment could intervene to call Mr. Safire off. In fact, Garment and Safire never communicated throughout the episode. Says Garment: "Bill, he's terrific; his mother wouldn't get the time of day if he's got a good story." Wick's invasions of callers' privacy were not serious, Garment says ("hundreds of tapes about things like Frank Sinatra's singing engagements"), so the lawyer's prescription was a large slice of humble pie. "There were investigations in seven states, and we went trooping around, apologizing to district attorneys, writing letters to all the people who had been recorded, calling Jimmy Carter, having a prayer session on the telephone." Wick, dutifully contrite, kept his job. In his handling of Wick's case, though, Garment was just doing tackling drills for his goal-line stands on behalf of Mr. Meese and Bud McFarlane. McFarlane, who described in immunized testimony before Congress how he had been instrumental in arranging the Reagan administration's arms sales to Iran, ultimately pleaded guilty to just four misdemeanors for withholding information from Congress. His sentence included no time in jail. In his book "Opening Arguments," Jeffrey Toobin, formerly an assistant prosecutor on Walsh's staff, writes that "the McFarlane plea bargain turned out to be one of the worst mistakes the [special prosecutor's office] made." Mr. Toobin adds that resentment toward Garment grew within the prosecution staff because of the defense lawyer's "relentless preening before the press." The staff dubbed Garment the "Spin Doctor." Toobin grudgingly admits, however: "We could mock Garment all we wanted. The Spin Doctor had taken us to the cleaners." The political litigation Garment is probably most proud of, though, is his defense of Meese during an investigation by a special prosecutor in 1984. Meese, then one of President Reagan's top White House advisers, was nominated to be attorney general that year. PRIOR to confirmation, however, Meese was accused of financial improprieties. Meese requested that an independent counsel examine the allegations, against the advice of many friends and most of Garment's law partners. They feared that Meese not only wouldn't be confirmed, but that he risked possible indictment and prosecution. The gamble paid off: The independent counsel, Jacob A. Stein, found no probable cause to indict Meese on any criminal charges, and the Senate confirmed Meese 63 to 31. Garment says that his client would have been treated more harshly in the current political climate, when Iran-contra, the Robert Bork and Clarence Thomas confirmation battles, and years of divided government have left relations between the executive and legislative branches more suspicious and bitter than perhaps ever before. Although he has thrived in the rough-and-tumble of political litigation, Garment, like his wife, has misgivings about the culture of scandal that has incubated in Washington. He has the defense lawyer's gut distaste for the enormous power of government being brought heavily to bear against an individual man or woman - especially from political motives. Beyond that, Garment fears that "political comity" has been wrung out of the American system of government. "Political life in a democracy requires elbow room, play in the joints," he says. "If we try to codify the complexity of American life and culture - make it all square corners - we're going to stumble into rigidity, anger, hostility. It will be different species talking to each other rather than a conversation within a common culture where there are differences that are resolved, negotiated, reconciled."