Billy Graham has learned a lot since the first time he visited the White House. He spent 30 minutes with Harry Truman, he said in an interview yesterday, and then was besieged by reporters on his way out. "I didn't know you were never supposed to quote the president, so I told them everything we said. Then they said they hadn't gotten a picture of the president and me praying, and asked if I would reenact it. Like a fool I went over and knelt on the White House lawn and posed. That picture was on the front page of newspapers all over the place."

It was the start of what has become a long and visible career of ministering to the powerful. He'll be returning to Washington next week, he said, for a prayer breakfast and will spend the night at the home of Vice President George Bush, "an old friend." He was here during the inauguration, saying a prayer at the church service attended by the Reagans. Graham, a registered Democrat, won't say who he voted for last November -- "I voted by absentee ballot in Tokyo. I stayed away until just a few days before the election. I came back to see Steve McQueen before he died, at his request."

Last night Billy Graham became art of the National Religious Broadcasters Hall of Fame, before a packed ballroom at the group's 39th annual convention. He spoke of the enormous success of the evangelical movement, of which he is a founding father, attributing it in large part to the growth of "electronic evangelism." This has become a growth industry that has spawned not only 1,400 Christian-owned radio stations and three television networks, but adjunct political lobbies as well.

Graham is not wholly enthusiastic about all this activity, and warned his fellow broadcasters of the dnagers of success in a speech to them last night. He stayed away from the much-heralded movement of conservative Christians into politics, exemplified by groups like the Moral Majority, because, he explained earlier, he did not think that some of the issues -- like opposition to the Panama Canal treaty -- were spiritual or moral questions. He is concerned that "you don't hear much [from them] about the hungry masses, the inner-city ghettos or the nuclear arms race." He turned down a request to address a "summit conference" the emerging forces held last summer, he said.

He's worried too that too many of the "Christian programs" on the air present a "caricature of Christianity," an overemphasis on grimmicky pleas for money.

"Instead of trusting God to supply our financial needs, we may have been relying on a sophisticated financial campaign . . ." he told the crowd last night. "What the evangelical movement does not need is a state-of-the-art idol." Later he quoted songwriter Irving Berlin, saying, "The toughest thing about being a success is that you have to keep being successful."

"When I started years ago [in 1949]," he said earlier, "I had to face the Elmer Gantry image. The emotionalism, the anti-intellectualism, anti-chruch, big money, money put in people's pockets." He consulted a friend, who advised: incorporate, and get a board of trustees. He did, and now almost every evangelist does the same.

Billy Graham is an American classic. For all his troubles over the years -- the Elmer Gantry image, being criticized for not condemning the war in Vietnam, remaining loyal to his friend Richard M. Nixon during Watergate, hassles with the IRS and the black community -- Graham is still the evangelist's evangelist, role model and international celebrity. New religious stars are heralded as "the new Billy Graham," and while he may be surpassed in total viewers or donations by some of his imitators, his television specials are admired and his "crusades" around the world are still setting attendance records.

He is now 62, and his blond hair is turning gray in a way that makes him look as though his roots were showing. His face is bronzed. He talks in a vigorous twang ( he lives in his home state of North Carolina), combining a steady flow of self-deprecating anecdotes with mentions of important names. He has a way of anticipating potentially unpleasant questions.

"I haven't announced this before, but the IRS did a 10-month audit of us last year and did not have a single suggestion in the letter they sent us," he said. "Our treasurer has been treasurer of Harvard for a number of years. We have 25 men and women on our board of directors, both black and white. . . We have the best lawyers and accountants in Minneapolis, Washington, New York and Atlanta, to be sure we don't have a penny out of place."

Billy Graham Evangelistic Enterprises, which includes a television production company and a movie company, has a budget of about $30 million a year, he said, all funded by donations. Graham's enterprises started making their accounts public only in 1977. "A lot of people did things after Watergate they had never done before," he laughed.

He was a small-town boy, referred to in The Charlotte Observer as "the son of Mr. and Mrs. W.F. Graham of Pard Rd." even into the 1950s, when he was becoming the best-known evangelist in the country. Graham spent three months at Bob Jones University but left because he did not agree with their brand of Christian "separatism." The Bob Joneses, father and son, look upon Graham as something close to an instrument of the devil.

Graham became president of a small college in Minneapolis, where his enterprises are still headquartered. He was asked to hold a series of "tent meetings" in Los Angeles, and it was there his career took off -- thanks to a flood of favorable publicity from William Randolph Hearst, who wired all his newspapers to "puff Graham."

"I met Hearst's son on a plane once, and I asked him, 'What happened?'" Graham recalled. "I'd never met his father or written him or had any exchange with him whatsoever. He said he'd heard a story that Hearst's maid had come to one of the meetings and told him about me, and that he had disguised himself and with Marion Davies come to see me. They like what they heard and felt that America needed a spiritual awakening."

From that point on he has been a media star. "I remember one headline in The Boston Traveler. I had a little cold, but the headline said "Graham Carries On in Spite of Death Threat!"

Graham has changed in more ways than his understanding of how to handle the media, at which he is expert. The nuclear arms race, for example. He now supports the idea of "SALT 10" -- the destruction of all nuclear weapons and biochemical weapons."

"I'd never thought it through before," he said of his change of heart (discussed for the first time publicly in March, 1979 -- on television). "We have the ability to destroy the whole world . . . we're spending $550 billion on nuclear arms around the world. It's already costing millions of lives because of the millions who are living on the knife edge of starvation. . . I'm not a pacifist. I think we have a right to defend our country. . . Jesus said when they came to capture him, and Peter pulled out his sword, 'If I were going to build a kingdom here on earth, my friends would fight for me. But I have not come to build my kingdom on earth by force'. . . We do have the hope that God is not going to let man blow himself up. But in the meantime we are to be peacemakers." He delivered a version of this message last night to a crowd known for its political conservatism. They applauded when he said firmly that he opposed total disarmament.There was only scattered applause when he talked about "SALT 10."

He remembers the '50s and '60s, when "I almost identified this country as the Kingdom of God." Now that his early saber-rattling has mellowed into what he describes as an amalgam of conservative, moderate and liberal views, he seems to view his younger colleagues with avuncular tolerance.

"It took me a long time to get to this point," he said.