With 39 parliamentarians, the Samajwadi Party may hold the trump card for the survival of the United Progressive Alliance government in case the Left parties withdraw their support to it in the event of the Centre going ahead with the Indo-US nuclear deal.

In the Lok Sabha, which has an effective strength of 543, the UPA enjoys support of 237 members -- 35 short of simple majority. The Left parties have 59 lawmakers as a block.

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Besides, there are parties like Bahujan Samaj Party with 17 members, Telugu Desam Party with three and six independents, who cannot be completely counted with the opposition as of now.

The SP, which has shown signs of warming up to the Congress, has not made its position clear on the nuclear deal. Its MP, Shahid Siddiqui had said on Thursday that the party would take a look if the government comes forward with any new logic or fact. But as of now, the SP is opposed to the nuclear deal with the US, he added.

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If the SP abstains from voting, then the Congress may be short of majority by about 15 MPs because of the reduction in the effective strength of the House. The ruling coalition may have to bank on smaller parties, independents and some fence sitters.

But in case SP votes with the government, the BSP may be expected to go against it.

If the Left also decides to abstain from voting on the issue, then the government may sail through easily because the half way mark also falls drastically.

Congress which has 153 MPs, enjoys support of Rashtriya Janata Dal (24), Dravida Munnetra Kazhagam (16), Nationalist Congress Party (11), Pattali Makkal Katchi (six), Jharkhand Mukti Morcha (five) and Lok Janshakti Party (four) besides others.

In the opposition NDA has over 180 MPs with BJP having 130, Shiv Sena (12), Biju Janata Dal (11), Janata Dal-United and Shiromani Akali Dal (eight each) and TDP five, besides other smaller parties.

Left parties, however, concede that the deal could go through and the government survive if SP supports it.

But, the Left parties still have not given up hopes and sources say they were trying to work with the government on how to reconcile the differences.

Communist Party of India-Marxist leader Prakash Karat on Friday hoped the government would last its full term and asserted that the Left was making all efforts towards that end.

CPI leader A B Bardhan has said the opposition to the deal was not a move aimed at bringing down the government but only to prevent it from implementing the deal.

Siddiqui said SP was opposed to the nuclear deal but if the UPA government came up with some new facts related to the deal, then 'we may take a decision which would be beneficial to the country'.

The decision would be above any political considerations, as it is something which is of national interest, he said, declining to elaborate.