To know why Left is opposing 123 deal, you have to go beyond what Newspapers or these Political people say.
US and India both acknowledge the threat of having hostile Pakistan and Strong China as Inda's neighbour.For obvious and other interests of US, It want to empower India to be able to keep check on both these countries. Nuclear power for peacefull purposes is totally shit thing. US is giving this name to resist opposition from its own people and world oppositon. China is totally aware of the situation and anybody who knows these Left parties can imaginge that China is thus bribing Left to oppose this deal so that this agreement is not signed. Left leaders are known traitors. Even if their doubts are true than what will happen if India conduct nuclear test and then who will stop them. At that time worst or worst US will withdraw their support and levey some penalties whcih will even in that case will be equal to today's situaiton and therefore India only stands to win in any siutuation after signing this agreement. Hope COngress Strategists are listening this argument!!!