The widow of former Prime Minister Rajiv Gandhi would have become India's first Roman Catholic prime minister had she not surprised everyone by turning the post down after her general election success in 2004.
She said her inner voice had dissuaded her from taking the top job, but she remained leader of the Congress party.
However, during the Congress-led rule of the United Progressive Alliance (UPA) her profile has increased and many Indians, including opposition parties, see her as a de facto ruler.
Her influence over the party has never been in doubt.
She is widely considered to be the main backer of policies including a more than $2bn (£1.4bn) food-for-work scheme often described as India's New Deal, and a landmark right-to-information law.
However, critics have questioned her political management skills and say she still depends on party managers to take key decisions.
They say that under her curious alliance with technocrat Prime Minister Manmohan Singh crises have been allowed to develop, including the failure to keep Communist allies on side while pushing through a nuclear deal with the US before winning a second general election in 2009.
Revered name
Before the surprise 2004 election win, Mrs Gandhi's future in Indian politics had looked somewhat uncertain.
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| Rajiv Gandhi was killed by a Tamil suicide bomber in 1991 |
After distancing herself from politics following her husband's assassination in 1991, Mrs Gandhi was initially seen as a reluctant and almost reclusive politician.
She officially took charge of the Congress party in 1998 and was elected to parliament in elections in 1999.
Under her leadership, the party turned in its worst performance since independence in the 1999 general elections. Congress also performed indifferently in state elections in 2003.
But the Gandhi name is still revered in India and Congress looked to Mrs Gandhi to translate that feeling into votes.
In 2004, her political opponents attempted to rake up her Italian descent as an election issue, saying the choice for voters was between an Indian or foreign leader.
But their appeal to xenophobia apparently fell on deaf ears.
Long before the election she surrendered her Italian passport in favour of full Indian citizenship.
Sonia herself said in a television interview: "I never felt they look at me as a foreigner. Because I'm not. I am Indian."
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| Critics have blamed India's ruling duo for poor coalition management |
Though she turned down the post of prime minister, Sonia is seen to have shed her formerly taciturn manner, routinely working the crowds at political meetings and displaying a more combative approach.
"We have the power and we could teach those anti-social forces a lesson," she told one meeting, alluding to the perpetrators of the deadly Mumbai attacks of November 2008.
"Our patience should not be treated as our weakness... We will not tolerate any act against India."
Sonia Maino was born on 9 December 1946 in the town of Orbassano, near Turin, to a building contractor and his wife.
She was raised in a traditional Roman Catholic household. Her mother and two sisters still live in Orbassano.
In 1964 she went to Cambridge to study English at a language school.
Her life changed forever when she met her future husband, Rajiv Gandhi, who was studying engineering at the university.
The couple married in 1968 and she moved into the house of her mother-in-law and then Prime Minister Indira Gandhi.
She initially disliked Indian food and clothes and caused controversy when she was photographed wearing a mini skirt.
But she spent the 1970s becoming steeped in Indian culture. Although she has learned Hindi, she is not a fluent speaker of the language.
She no doubt also watched and learned as Indira fought a variety of political battles.
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| Rahul (left) and Priyanka Gandhi have also entered the political fray |
Sonia was propelled into the forefront of the Indian political scene as Rajiv, whose brother Sanjay died in a plane crash in 1980, was picked as the successor to the Gandhi-Nehru crown.
Rajiv became prime minister but seven years later tragedy struck the Gandhi family again.
Sonia's husband was killed by a Tamil suicide bomber during a visit to Madras (Chennai) in 1991.
She and her children, Rahul and Priyanka, were consumed with grief.
Sonia resisted Congress attempts to persuade her to step into Rajiv's shoes and eschewed politics for several years.
Eventually, in 1998, she agreed to become more involved but her initial efforts were overshadowed by Congress's humiliating defeat by the BJP in the 1999 election.
In August 2000 she became a grandmother when Priyanka gave birth to a son.
Earlier that year Sonia had asked a court to grant clemency to a woman who had played a part in the bomb attack that killed her husband.
The bomber, Nalini, had appealed for mercy on the grounds that her seven-year-old daughter would be orphaned if she was hanged.
The court later commuted Nalini's death sentence.
Sonia is a familiar figure in Amethi, her husband's rural parliamentary constituency in the northern state of Uttar Pradesh which her son Rahul now represents.
She herself has represented the neighbouring seat of Rai Bareilly.
She suffered a minor setback in 2006 when she had to surrender her membership of parliament, following allegations that she was violating rules by profiting from a second public post.
She won a by-election later in the year to return to parliament.
But her ascendancy over her party since it won power proves how much Congress still depends on the Gandhi dynasty.
