New Delhi — It will be the largest election ever, anywhere in the world. On Dec. 24 and 27, Indian voters - potentially 390 million people - go to the polls. But the campaign is already being treated with a certain degree of nonchalance. Rajiv Gandhi, who completes his first month as prime minister on Saturday, the day the campaign begins, is expected to be comfortably returned to power.
Thus will continue the Nehru dynasty, spawned by Mr. Gandhi's grandfather, Jawaharlal Nehru, and continued by his mother, Indira Gandhi, until her killing last month.
It is only after the election, in the view of many, that Mr. Gandhi's real political tests will come.
A number of factors have lent a certain degree of the unnatural to the coming campaign: the shock of his mother's assassination, the swiftness with which he called the parliamentary polls, and the relative ease with which he, at least for now, consolidated his hold over the family's fractious Congress (I) Party.
The sympathy vote for Gandhi, a most reluctant entrant into the political fray, is thought to be quite substantial. And, although the elections were called only a month ahead of schedule, the seven nationally based opposition parties, plus 31 from individual states, were caught unaware and napping. They remain in a state of seemingly irreparable disarray.
The three major noncommunist opposition parties or alliances were holding their ''final unity talks'' Wednesday even though they were still unable to agree on mutual compromises - especially over which seats each of them will contest in the critical northern Hindi belt. Since the deadline for filing nomination papers had already expired Tuesday, the meetings seemed specious.
The Hindi-speaking heartland, which envelops the Ganges River and the country's northern plains, has in the past decided India's elections. It holds 220, or 40 percent, of the nation's parliamentary seats. Despite the failure to reach agreement in time on mutually acceptable candidates for this huge swathe of the country, opposition optimists were still hoping to agree on common candidates in nearly 300 of parliament's 515 contested seats.
''It's rather fanciful,'' said one Western official, ''to hear them speak of fielding common candidates everywhere but in the Hindi belt. They don't have any presence anywhere except the Hindi belt.''
Thus, the pattern of past elections could well be repeated in India's eighth national election since independence 37 years ago. The ruling Congress Party, only once defeated in any national poll - in 1977 - could win an overwhelming majority with substantially less than 50 percent of the vote.
In past contests, the party has only captured 35 to 45 percent of the vote, but has swept to power because the opposition has been so badly split. And, with four-candidate contests predicted in the Hindi belt, said one disheartened opposition leader, it will ''really be more of the same. The opposition will get more than 50 percent of the vote, as we always have. But it will be split, and the Congress (party) will be the victor . . . really, just as it's always been.''
The opposition parties have received a dual setback:
* Indira Gandhi's assassination left them bereft of any substantial electoral plank. Before her death, the parties' campaign strategy had been predicated upon attacking Mrs. Gandhi's authoritarian ways - her overcentralization of power, toppling chief ministers and state governments apparently at will.
* And Rajiv Gandhi, in consolidating his hold on power, however tenuous it may prove, has moved deftly in a number of compromise moves. He has therefore neither greatly pleased nor greatly displeased any of the Congress Party's powerful and conflicting interests or clans.
Only 83 of the party's 336 parliamentary incumbents have been removed as candidates. (The Congress holds 351 seats in parliament, but elections will not be held in the troubled Punjab or the state of Assam.) This is less than half of those recommended for purging by Gandhi's closest advisers and aides.
But the 83 have touched upon all political areas Gandhi vowed to ''clean up'' - the most corrupt and inefficient, the most intolerable, and the most obsequious of his mother's sycophants. They also include a handful of the more unethical proteges of his late brother, Sanjay, including two who were identified by scores of witnesses as being involved in the anti-Sikh carnage after Mrs. Gandhi's death at the hands of two Sikh bodyguards.
''It's nothing extraordinary,'' said one Western official, ''but perhaps he's given a signal of things to come.''
Such observers say that in the limited time available, the inexperienced Mr. Gandhi has shown an impressive coolness, determination, and, on most occasions, realism and solid good sense.
There have been some lapses, however, in the eyes of diplomats. But Indians consider these lapses as pluses in a crucial election year. During his first mass rally as prime minister, to commemorate his mother's 67th birthday last week, he charged that there was a ''conspiracy'' behind Mrs. Gandhi's death with ''certain elements active in foreign countries.''
He offered no specifics. Nor did he indicate whether he referred to Sikh separatists living abroad, or to foreign powers, and, if so, to whom.
''A rather sad and depressing maiden address,'' said a Western diplomat. ''Once again, evoking the foreign hand is simply playing his mother's drum. . . . And the fact that he never mentioned the post-assassination mayhem against the country's Sikhs showed no recognition of their suffering or estrangement. . . . It's really rather depressing when you consider all that's gone on.''