A deft, relaxed Gandhi hits his stride in Indian election contest

''Indiraji, I'll vote for Indiraji,'' the bent-over, toothless old woman repeated as she sat cross-legged in the hot and dusty vegetable market. Few can afford stalls, so their meager produce lies haphazardly on the dried-out ground.

When told that ''Indiraji'' - a respectful term for Indira Gandhi - had been assassinated, she doggedly said, ''That doesn't matter. I'll vote for her anyway.''

Beyond her was the incongruity of bullock carts getting entangled in Christmas tree lights, of tinsel hanging from appalling slums, of election banners fluttering aimlessly. All of Amethi - women in dirty saris, men without shoes - waited amid the glitter for Wednesday's visit by Rajiv Gandhi to his parliamentary constituency, the first since he became prime minister.

He drove through ceremonial arches in a bulletproof car. He was relaxed, almost boyish, as he talked freely with carefully screened members of the crowds. It was a security man's nightmare, but for Mr. Gandhi it was a conscious effort to impress upon his constituents, some of the poorest and most ragged in all of India, that he had not changed since becoming premier, that he had not forgotten Amethi. During scores of earlier visits, he had always driven himself.

But even here in Amethi, there has been a rather tired element to the election campaign, which was robbed of its single issue when Mrs. Gandhi was assassinated Oct. 31: Should Indira Gandhi rule India, or should she not?

The drama of the battle between Mr. Gandhi, and his sister-in-law, Maneka Gandhi, who is challenging him for his parliamentary seat in the Dec. 24 polls, has also lost much of its dynastic intrigue. The freckle-faced former model, who before the assassination was believed capable of giving Mr. Gandhi a substantial challenge for his seat, has lost much of her momentum.

Officials from the ruling Congress (I) Party concede, however, that she could embarrass the prime minister by reducing the margin of his victory in the Amethi race. In June 1981 he won 84 percent of the vote in the by-election to fill the seat held by his brother, Sanjay, who had been killed in a plane crash.

This time, all the opposition parties are supporting Maneka. So, except for a handful of independents, it will be a straight race between the man who is called the ''prince of India,'' and the woman dubbed the ''usurper,'' challenging him for what she calls her ''rightful place.''

Rajiv Gandhi covered much of the constituency's 687 square miles in a two-day whirlwind tour. He delivered essentially the same speech he has delivered since the campaign began. Its message: A vote for Congress (I) will take India into the 21st century, and it is the only party that can guarantee the country's national unity and integrity.

He was also asking, by extension, for a vote of confidence in the Nehru-Gandhi dynasty, which has ruled this kaleidoscopic nation for 32 of its 37 independent years.

He showed a deftness that was absent before. He was more relaxed before large audiences - though they were not so large as the party had hoped. He laughed easily and occasionally even joked.

On the question of streamlining the country's extraordinarily sluggish bureaucracy, he told reporters there would be ''no political interference in the bureaucracy, and no bureaucratic interference in politics.'' He paused, then added, ''except where political guidance is necessary.'' It was a deftness in covering all options that he had not often shown.

He has gained a reputation in his district for pressing hard for centrally funded public-works projects and small-scale industries.

Posters of Indira Gandhi - and smaller ones of her son - hung on the peeling walls of the old town of Jagdishpur and throughout the Amethi constituency. There were occasional posters of Maneka, holding her son Varun and flanked by her late husband, who had been Mrs. Gandhi's chosen successor.

Chowdry Azhar Hussein, a friend of Sanjay's who is heading Maneka's campaign office in Jagdishpur, an electorally significant Muslim town, said: ''The sympathy factor (from Indira Gandhi's assassination) is fading. And what about sympathy for Maneka? She lost a husband, was then thrown out of her mother-in-law's house.'' Mr. Hussein was sounding a theme of Maneka's campaign that strikes a responsive chord in Indian women.

He predicted that Maneka's party, the National San-jay Platform, would win half the 55 seats it is contesting in Uttar Pradesh State. There are 85 seats in the state. Other poll-watchers are predicting the ruling Congress (I) will capture between 45 and 55 of the state's seats.

''They may win or lose a few,'' said a professor from Lucknow, the state capital, ''but the main thrust of their statewide campaign is to reverse their pre-assassination slippage and hold on to their present 51 seats.''

As dawn broke in Lucknow over the sacred river Ganges, monkeys leaped from trees, water buffalo moved through fields, and white-bearded men wearing waist-to-knee dhotis, prayed at the river's banks.

It was a scene that has been repeated for thousands of years, through Hindu and Mogul empires in Uttar Pradesh.

Later in Amethi, one was rudely thrown back into the 20th century as bullock carts, cycle-rickshaws, and wandering sacred cows jostled for position in the impoverished center of Amethi.

In the faded elegance of his 250-year-old palace, the former Maharaja of Amethi's once princely state, Raja Rananjaya Singh, a scion of the 1,000 -year-old Rajput dynasty, said with certainty, ''Rajiv Gandhi will win.''

His message was repeated by many of the landlords who control vast areas and vast electorates across the poor and backward reaches of Uttar Pradesh.