Its decision to end its backing is expected to lead to a vote of confidence in the parliament in the next 15 days, Gopalan said.
But newly enlisted support from the regional Samajwadi Party should enable Manmohan Singh, the prime minister, and his Congress-led government to win the ballot and avoid calling early general elections.
"I don't think it will affect the stability of our government," Singh said on Tuesday while attending the Japan summit of the Group of Eight industrialised nations.
Singh is expected to discuss the nuclear agreement with Bush at the Japan summit on Wednesday.
The leftists announced their withdrawal after Singh said the government would be meeting with the UN's International Atomic Energy Agency (IAEA) soon.
India needs to win a waiver from the IAEA before the US congress can approve a pact that would bring India into the fold of global nuclear commerce after being shut out for decades.
|Both US and Indian leaders are eager to make a deal [EPA]
The deal would make India eligible for imports of nuclear fuel and civilian nuclear technology, a right usually reserved for signatories of the Nuclear Nonproliferation Treaty - which India refused to sign as it developed its nuclear weapons in secret.
The leftist parties said the deal would bind India too closely to the US and that allowing UN inspections of the country's civil nuclear programme - as demanded by the Americans - would harm India's strategic weapons programme.
Both India and George Bush, US president, are eager to push the pact through during his tenure.
On Tuesday, Dana Perino, White House spokeswoman, spoke of a narrowing window of opportunity to finalise the deal before the November 2008 presidential polls, pointing to a heavy workload and "a limited number of legislative days" for the US congress.