WATCHING HIM address a joint session of Congress, you had to believe in genes. Here was the grandson of Jawaharlal Nehur, the son of Indira Gandhi, all poise and calm in his grey Nehru suit, his diction flawless, his syntax polished, confidently telling the world of his ideas and plans.

Rajiv Nehru may not have been brought up to be prime minister of India -- his younger brother Sanjay was to have inherited the mantle -- but he was, it seems, born to be.

He combines the breathtaking self-assurance of a Brahmin with the nervous system of an airline pilot, his work before he was drafted to succeed his assassinated mother. Washington was terribly impressed from the moment he arrived.

His grandfather irritated some Americans who found him lofty and self-righteous. His mother was thought to be high-handed. They both had the Indian tendency to moralize, an activity the new representative of the family does not have time for.

John Foster Dulles, President Dwight Eisenhower's secretary of state, was driven to near apoplexy by Nehru's policy of nonalignment. He called it "immoral." Richard Nixon, who simply could not stand Indira Gandhi, "tilted" toward Pakistan, India's undemocratic but anticommunist neighbor and archrival.

Rajiv Gandhi subscribes to the family philosophy on nonalignment, and, before Congress, made no bones about or apologies for.

Yet Ronald Reagan, the most anticommunist American president of them all, was seemingly much taken with his Indian guest. He took pains, after their first meeting, to put out the word that they had "hit it off."

Reagan is usually adamant about nations choosing sides. Why was he so cordial to someone who stubbornly refuses to share his view of the Soviet Union as the source of all evil in the modern world?

Possibly Reagan, who is, under his genial skin, one of the most competitive politicians on the face of the earth, may have wanted to show Gandhi that he is infinitely more charming than Gandhi's first foreign host, Mikhail Gorbachev. On the other hand, he may have recognized instantly Gandhi's star quality and made up his mind to be nice.

Of course, he was greeting a man with a mandate more sweeping than his own. Gandhi, his first time out, campaigned -- and won -- clearly on his name, which is the most powerful in India. Even so, he brought the Congress Party home with a majority of 520 to 401.

Reagan's disapproval of the fact that his guest's first trip out of India as prime minster was to Moscow had been registered. Gandhi had been quizzed about it during his visit with Gorbachev. He turned aside the query with an airy rejoinder. "They asked me first," he said. A Gandhi is not easily fazed.

When challenged about his country's evenhandedness between the superpowers, Gandhi is matter of fact. India tried to buy weapons from the United States and had such intolerable conditions put on the purchase that it decided to go elsewhere. It was the same with heavy industry. The Soviet Union was easier to deal with.

Reagan is, as Daniel Ortega of Nicaragua can testify, exceptionally severe with Third Worlders who, when turned down by the United States, go east for their necessities, but Gandhi escaped censure.

We must remember that Reagan has his trendy side, and right now India is raging chic in the United States. Two British films, "A Passage to India" and "Jewel in the Crown," opened American eyes to the subcontinent. Gandhi brought with him the gorgeous treasures of the Festival of India, a dazzling and opulent display of the art, music and dance of an ancient culture.

Americans derived much of their consciousness of India from Mother Teresa, the heroic nun who picks up the dead and the dying paupers on the streets of Calcutta. The festival, which was a smash before it opened in many U.S. museums, has given a new dimension to Americans' idea of India.

Gandhi made it known in a mannerly way that he does not feel the United States has all the answers. He was extremely focused and sceptical about the technological wonders shown to him at the National Academy of Sciences. How could they apply to India, he asked. If they did not, he passed on.

Despite the preoccupations which all but engulf the leader of a nation of 740 million people, with acute development needs and the menacing problem of a rebellious minority, the Sikhs, Gandhi told the Congress he will carry on the family tradition of playing a part in the global game.

He is a genuine new personality on the world scene, and he is only 40 years old. He has tremendous appeal for the youth. The House chamber was packed with young people. On Dec. 31, he was sworn into a job that has been in the family for 40 years and that was handed to him. Unexpectedly, already, he has shown signs that he was made for it.