Mr Singh has promised the government will prove its majority
Indian MPs are holding a vote of confidence in the Congress party-led government that could decide the fate of a nuclear deal with the US.
It comes after the government's left-wing allies withdrew their support in protest against the controversial civilian nuclear accord.
Observers say the vote following two days of debate is too close to call.
If the government loses the vote, India faces early elections and the nuclear deal would be under threat.
The BBC's Chris Morris in Delhi says the momentum appeared to be with the government as the second day of parliamentary debate began, but it is extremely tight.
HAVE YOUR SAY
Whether the government stays in power or not, it has lost the credibility and confidence of people at large
Rakesh Punia, Delhi
Late on Tuesday afternoon, Speaker Somnath Chatterjee adjourned the debate amid allegations of vote-buying.
There was uproar when opposition Bharatiya Janata Party (BJP) members waved fistfuls of money in the air, alleging that they had been offered bribes to abstain.
Mr Chatterjee called it a "very sad day" for the Indian parliament, adding: "Nobody will be spared if found guilty."
Members of parliament have been summoned from their sick beds and even from prison cells to take part in the vote.
TIMETABLE FOR NUCLEAR ACCORD
Approval needed from IAEA, expected to meet on 1 August
Consent also required from 45-member Nuclear Suppliers Group
Congress to approve deal before President Bush signs it into law
All this to happen before Mr Bush's tenure expires on 19 January 2009
A BBC correspondent in the lower house of the parliament, where the debate is being held, says it is packed to capacity.
The visitors' gallery is also full with members from the upper house, businessmen, diplomats, and local and international media present.
Federal railway minister Laloo Prasad Yadav had the MPs and visitors in splits during his speech with his jokes and wisecracks aimed at the opposition.
Analyst Prem Shankar Jha said it was difficult to predict who would win the vote of confidence.
"So hectic has been the horse-trading been that it is no longer possible to predict what will happen," he said.
Prime Minister Singh promised the government would "prove its majority" before the debate began on Monday.
Federal finance minister P Chidambaram defended the nuclear deal during the debate on Tuesday.
"The question is do we want to come out of nuclear isolation? I want India to become an economic power, a superpower," he said.
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 Indian media is predicting a close finish
The communists fear the accord could give the US too much influence over Indian foreign and nuclear policy.
The main opposition Hindu nationalist BJP fears that the deal could compromise India's ability to test nuclear weapons in the future.
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 has 153 MPs. The BJP has 130.
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.
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 be 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.