Sources close to the Gandhi family say she has been strongly influenced by her late mother-in-law, former prime minister Indira Gandhi, whose policies were considerably left of centre.
Sonia Gandhi initially declined the throne of the Congress party after the 1991 assassination of her husband and former prime minister, Rajiv, but she finally agreed to enter politics six years later to lift the flagging fortunes of the party under her family's brand name.
As party president and chairperson of the ruling coalition, she has kept a low and almost enigmatic profile, appearing to stand above the political fray.
When she went abroad for surgery last year there was no official word on her illness, and the media tamely accorded her the privacy a royal might expect.
But Gandhi wields extraordinary power behind the scenes, shaping policy out of the public eye with a tight circle of decision-makers and relying on back-channel negotiations to manage relations with fractious coalition allies.
Ratings agency Standard & Poor's, warning earlier this year that India could become the first of the emerging market BRICS economies to lose its investment-grade rating, took a swipe at her resistance to policies mooted by Singh.
"The Congress party is divided on economic policies. There is substantial opposition within the party to any serious liberalisation of the economy," S&P said.
"Moreover, paramount political power rests with the leader of the Congress party, Sonia Gandhi, who holds no cabinet position, while the government is led by an unelected prime minister ... who lacks a political base of his own."
Image: Indira Gandhi seen with sons Rajiv Gandhi and Sanjay Gandhi in this file photo.