The world's largest democracy has been stunned by the size of the upset poll win by Gandhi's Congress over Prime Minister Atal Behari Vajpayee, who was rejected by the disaffected rural poor angry at missing out on the benefits of India's economic boom.
"Shock and awesome" said the Hindustan Times in its front page banner headline after Thursday's national election count; "King Cong, Queen Sonia", said The Times of India.
But Gandhi is still considered a political novice after replacing her husband, former prime minister Rajiv Gandhi, as Congress chief only seven years after his 1991 assassination.
Written off by opinion polls just three weeks ago, Congress fared far better than expected and will be the largest party in the new 545-seat parliament. But Gandhi's coalition, with fewer than 220 seats, needs new partners.
She now faces the delicate task of stitching up an alliance with leftist parties, which hold a critical bloc of 60-plus seats but which are opposed to the style of economic reforms introduced by Vajpayee's Bharatiya Janata Party (BJP) to open the economy.
The leading left-wing party, the Communist Party of India (Marxist), won 33 seats, more than half the leftist total, and its support will be critical to the survival of a Congress-led government.
Reforms and privatisation
Communist leaders say they are not opposed outright to all reforms. But they do oppose privatising profitable state firms and want state workers consulted more, raising concerns about the pace of privatising India's inefficient and monolithic state enterprises.
But it was Congress who broke India out of socialist-style economics more than a decade ago and the party has pledged to press ahead with the reforms needed to make Asia's third-largest economy an economic superpower to rival China.
Rural communities largely
rejected the BJP
"I think if you look at the broad trajectory of the Indian economy, it has been unbroken by changes in government," Craig Barrett, chief executive of Intel Corp, the world's dominant chip maker, said.
Without a clear majority for her own coalition, Gandhi also needs communist support for her to become prime minister rather than hand the job to a compromise candidate.
Congress MPs meet on Saturday to choose their parliamentary leader, who would be expected to head the new government. After the size of her win Gandhi is the front-runner.
Although the BJP campaigned heavily on Gandhi's foreign origin, voters overwhelmingly rejected this as a concern. Nor have the leaders of smaller parties so far raised it in as an issue in forming a government over the next few days.
"The clearest message from the voters is that Mrs Gandhi's foreign origin is not - and has not been - an issue," said the Hindustan Times editorial. "There are no doubts that the prime ministership of the country is Mrs Gandhi's for the taking."
Gandhi's victory marks the revival not just of Congress, out of power since 1996, but of the country's first family, the Nehru-Gandhi dynasty that led India to independence in 1947 and ruled unchallenged for decades after.
Vajpayee resigned following his
party's poll defeat
Sonia's son, Rahul, stood for the first time and easily won his seat in the family borough in the "cow belt", the northern heartland of the overwhelming mass of India's rural poor.
Hectic parish-pump campaigning by Sonia, Rahul and Rahul's sister, Priyanka, who were mobbed wherever they went, played a major role in reviving the party's fortunes.
After rising to power on the back of a strong pro-Hindu campaign, 79-year-old Vajpayee put that on hold to seek the political centre ground with a focus on prosperity and peace when he called the election six months early.
But most of India's hundreds of millions of rural poor have seen no gains from surging economic growth, cheap loans and a liberalised economy with its flood of imported consumer goods.
That benefited mainly the urban middle class. For hundreds of millions of villagers, clean water, medical care, electricity, jobs and even enough food remain as much out of reach today as they were several years ago.