If the Nuclear Deal is pushed through, Congress will have enough grist for their election mill and they need it most given the dismal performance so far; if the deal fails, both Congress and the Left will lost no time to become comrades again as no party wants to lose power nuclear or political. Most probably, the trust vote may not take place and Cong. would backtrack on nuclear deal to stay in power and the left would also probably unabashedly support them then.
