The arrangement is odd, the Communists are not formally a part of the government, but they have supported the government on confidence votes, but they are now withdrawing their support because of the US-India nuclear deal, which they feel violated Indian sovereignty.
They don’t object to the nuclear cooperation. They object to signing the NNPT and negotiating with the IAEA.
It looks like both the Bush and His Evil Minions™ and the Indian Congress party are rushing to get this deal signed before January 21, 2009.