There is nothing wrong if CPI(M) vote along with the BJP. They did that before. in 1977 Morarji Desai government has survived because the LEFT was supporting it from outside, when Vajpayee was the foreign minister.
This is a national issue: (a) whether India would retain nuclear weapons to give up to satisfy American demand; (b) whether India will retain an independent foreign policy or it will be reduced to Thailand or Kenya.
India gains nothing from this 123 Dances; American Energy companies will gain. Just like ENRON , they will come to India to offer their junk nuclear plant at a high cost and just like ENRON they will run away without producing anything or just like UNION CARBIDE kill several thousands people.
The Strategy of the LEFT: LEFT want The Congress to be defeated. After that The Congress will discard MM Singh and call for a new election. After the election LEFT will again support The Congress without MM Singh. Most Congress leaders are against MM Singh; they will be happy to see him go after his humiliation. That is what also CPI(M) wants.