A communist leader said there would be early elections if the Congress-led government did not pull out of negotiations by the end of the month.
The government is due to hold fresh talks with the UN's nuclear watchdog, the IAEA, over the controversial deal.
Its left-wing allies have opposed the accord, saying it will give the US influence over India's foreign policy.
Prime Minister Manmohan Singh has defended the deal, which would allow India access to US nuclear technology and described it as a "landmark".
Last month, his communist allies agreed to talks with the IAEA.
But on Sunday the leader of the Communist Party of India (Marxist), Prakash Karat, told reporters that the government should stop further negotiations.
"If they still go ahead, we will have to be prepared for mid-term [general] elections," he said.
If the 60-odd communist MPs withdraw their support, India will face early elections.
Mr Karat's remarks came on the eve of Indian officials leaving for Vienna for a second round of talks with the IAEA.
It is the latest stage in the approval of the agreement, which has run into serious domestic opposition in India.
It has already been pronounced dead by some analysts, because of domestic political rivalries.
India's communist parties say they retain the right to reject the deal, even after the IAEA has examined it.
Mr Singh told US President George W Bush in October that he was having difficulty implementing the deal.
The deal has also been criticised by many outside India.
It would allow India to reprocess spent nuclear fuel - something that is opposed by some members of the US Congress, as India has not signed the nuclear Non-Proliferation Treaty.
Published: 2007/12/10 06:55:19 GMT
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