The Indian parliament is debating a vote of confidence in the Congress party-led government that could decide the fate of a nuclear deal with the US।
The vote comes after the government's left-wing allies withdrew their support in protest against the controversial civilian nuclear accord।
Observers expect that Tuesday's vote following the debate will be close।
If the government loses the vote, India faces early elections and the nuclear would be under threat।
Prime Minister Manmohan Singh says the government will "prove its majority"।
He told parliament he had always acted in the interest of the nation।
"Every single decision, every policy and initiative we have taken, was taken in the fullest confidence that we are doing so in the best interests of our people," he said।
Launching the debate, the leader of the main opposition Bharatiya Janata Party (BJP), LK Advani, said: "We are not against nuclear energy। We are not against a very close relationship with America.
"But we would never like India to become party to an agreement which is unequal।
"This deal makes us a subservient partner। It makes India a junior partner."
Under the accord, India, which has not signed the nuclear Non-Proliferation Treaty, would gain access to US civilian nuclear technology and fuel।
In return its civilian nuclear facilities would be opened to inspection। Nuclear weapons sites would remain off-limits.
The communists fear the accord could give the US too much influence over Indian foreign and nuclear policy।
After days of political bargaining and arm-twisting, the government and the opposition have headed into the debate still uncertain about who will win, the BBC's Chris Morris in Delhi says।
With the left withdrawing support, the government can rely on only 226 members in the 543-seat parliament, and needs 46 more to be absolutely sure of a majority।
Congress and the BJP have a nearly equal number of seats, with 153 MPs and 130 respectively।
The Congress party hopes it will get the backing of the regional Samajwadi party and other smaller parties to help it win।
India's media is awash with reports of alleged defections and desertions among MPs ahead of the vote।
"It's snatch-and-dash to the finish," headlined The Times Of India newspaper।
"Vote looks neck and neck," said The Asian Age। "Contest tightens, situation volatile," reported The Hindu.
Communist leader Prakash Karat said the government should junk the nuclear deal if its loses the vote।
"We wish to warn the government that in case you lose the trust vote, don't try and push the nuclear deal। The country will revolt," he said.
The opposition opposes the deal for a variety of reasons।
Indian communists simply want no partnership at all with the United States, while the Hindu nationalist BJP fears that the deal could compromise India's ability to test nuclear weapons in the future।
And smaller parties, on whom both government and opposition leaders have suddenly lavished attention, have been trying to calculate which side of the political divide suits them best।
India is under pressure from Washington to sign the accord before the US presidential election in November।
Last week, Indian officials met members of the International Atomic Energy Agency (IAEA), the world nuclear regulatory body, in Vienna to discuss plans to safeguard India's civilian nuclear facilities।
The IAEA's approval of the plan is a key condition for enacting the deal।
If the IAEA signs the agreement, the deal will go to the 45-nation Nuclear Suppliers Group, which regulates global civilian nuclear trade, for approval।
It must then by approved by the US Congress before President Bush can sign it into law।
Critics of the deal fear assistance to India's civil programme could free-up additional radioactive material for bomb-making purposes.
