The growing scandal over telecoms licences is one of the most serious challenges since the Congress party came to power in 2004.
The opposition has disrupted business in parliament since Nov. 9, demanding the Congress party-led coalition government agree to a full parliamentary investigation into alleged corruption in the telecoms ministry.
That has weakened the government's ability to move key economic measures and delayed legislation, although the government is not at risk of collapsing.
"I think it is shameful that a person of the integrity of the prime minister should be targeted in this manner. Everyone knows that the prime minister is 100 percent above board. Everyone knows what kind of an individual he is," Gandhi told reporters in her first comments on the scandal.
Telecoms Minister Andimuthu Raja was forced to resign more than a week ago after a state audit accused his ministry of selling licences and spectrum too cheaply in 2007-2008, potentially depriving the state of $39 billion in revenues.
"The minister has been asked to resign," Gandhi said. "When we compare the actions that our party takes with the actions that other parties take, as you have mentioned... I am confident that the people will judge us more positively than other parties."
Sonia, widow of former Prime Minister Rajiv Gandhi, is considered India's most powerful politician and effectively runs the government in coordination with the cabinet. She has been criticised for not taking a more public approach more often.
Singh's representative, the attorney general, defended the prime minister in court on Tuesday, saying all procedures had been followed in ordering a probe into the scandal. Singh will not personally face any sanctions but negative conclusions from the court could add to pressure on him to resign. Prime Minister Singh has been asked by the Supreme Court to provide more information after defending on Tuesday his delay and eventual failure to approve prosecution of his telecoms minister for selling telecoms licences below market price.
The opposition says Singh failed to crack down on corruption because he did not want to upset the DMK, which is key to ensuring the coalition government has a majority in parliament.
Raja is a member of the DMK, a small party from Tamil Nadu.
Congress has been mired in one controversy after the other since its strong re-election in 2009, being forced to sack several high ranking officials over alleged corruption and finding it difficult to pass key economic and political reform needed to lift millions out of poverty and open up the economy.
Critics say the affair highlights Singh's weak leadership skills which have often meant Finance Minister Pranab Mukherjee stepping in to handle problems.
(Reporting by C.J. Kuncheria; Writing by Paul de Bendern, Editing by Jo Winterbottom and Sugita Katyal)
