BY A TOPHEAVY margin never achieved either by his grandfather or mother, Rajiv Gandhi has become the elected successor to the political dynasty of Jawaharlal Nehru and Indira Gandhi. Prime Minister Gandhi wiped out the opposition and put himself and his Congress (I) party -- the "I" is for Indira -- in a position to ensure both continuity and reform in a country left numb by the assassination of Indira Gandhi scarcely two months ago.

How is it that a little-tested 40-year-old former airline pilot could, for the first time, lead Congress to more than half the total vote and take eight of every 10 seats in the governing house of Parliament? A sympathy for Indira Gandhi and for her son's loss of his mother no doubt was a large consideration. Rajiv Gandhi also encouraged and profitted from a tendency on the part of alarmed and patriotic Indians to portray those opposed to him and his mother and Congress as tampering with the unity and integrity of the country.

The opposition helped by coming before the electorate as a collection of proven failures given over to infighting and personal ambition. By contrast, Rajiv Gandhi looked poised and decent, certainly not corrupt, cool and caring in the Bhopal crisis. Keeping the elections on their scheduled track, despite his mother's murder and its terrible aftermath of popular vengeance against Sikhs, underlined his national credentials.

Mr. Gandhi's first priority must be to tend to grievances that produced the Sikh separatist uprising in the Punjab last June. He tried, during his campaign, to be a truly national figure and not to exploit the Hindu backlash stirred against Sikhs by the assassination of Mrs. Gandhi by two of her Sikh guards. Voting was not permitted in the Punjab, however -- or in a second strife-torn state, Assam. By moving quickly to restore normality and release Sikh prisoners, Prime Minister Gandhi can advance the necessarily unending process of making the world's most populous democracy, ethnically and sectionally riven as it is, a more perfect union.

The foreign policy corollary for Mr. Gandhi is to move beyond any temptation to bash Pakistan for its alleged -- unproven -- role in aiding Sikh terrorists. Tension between the two south Asian states needs to be settled down. That might help make it possible for India's new leader to consider whether he would not like to take some of the Moscow tilt out his mother's policy of nonalignment.