FACTBOX: Key Indian politicians

(Reuters) - The following are profiles of politicians who play an important role in the fate of the nuclear deal and Prime Minister Manmohan Singh's government, which faces a vote of confidence in parliament on Tuesday.

SONIA GANDHI, president of India's ruling Congress party: As the torchbearer of the Nehru-Gandhi dynasty, the 61-year-old widow of Prime Minister Rajiv Gandhi is seen as kingmaker in her party. She gave up the chance to become prime minister in 2004, earning the respect of many Indians as a selfless politician.

Voted the world's sixth most powerful woman by Forbes, she remains in charge and the party's main crowd-puller.

MANMOHAN SINGH, prime minister: The father of India's economic reforms, Singh was a compromise candidate for the prime minister's job when Sonia Gandhi turned it down.

The complex leadership structure with Gandhi at the top has been criticized as undermining the office of prime minister and the opposition has labeled Singh as a weak, directionless leader.

Yet with the nuclear deal, Singh, 75, put his foot down to support a pact that may be one of his few lasting legacies.

As finance minister in the early 1990s, he promoted free market economic reforms.

LAL KRISHNA ADVANI, the prime ministerial candidate of India's main Hindu nationalist opposition: Variously described as a hardliner, a hawk and a wily politician, Advani has pushed his trademark Hindu revivalism with the Bharatiya Janata Party (BJP).

Advani is credited with the upswing in his party's fortunes after it won a string of state elections since late last year.

Born, like Singh, in Pakistan when it was a part of undivided India, he is hawkish about ties with Islamabad.

When Advani's own coalition government was in power in 1999-2004, it pushed pro-market reforms, including privatization of state-run companies. But it was also accused of fanning tensions between Hindus and Muslims.

With record inflation and signs of economic slowdown, his party could benefit most from early elections.

PRAKASH KARAT, general secretary, Communist Party of India (Marxist): The communists have strongholds in three Indian states. Karat is their ideological hardliner.

Educated in politics at the University of Edinburgh, Karat grew from being a student leader to holding the party's top post. He is uncompromising in his opposition to India's growing ties with the United States. Any government propped up by his party will find it difficult to push economic reforms.

MULAYAM SINGH, head of the Samajwadi Party: With 39 federal lawmakers, this former wrestler now holds the key to the survival of the government after the withdrawal of the communists.

His party is based in the northern state of Uttar Pradesh and relies on support from the Yadav caste and Muslims.