Nadler:

"The President has an obligation under the Constitution to seek the approval of Congress for any use of military force unless there is an imminent threat to the United States or its allies.  Congress and the nation must have the opportunity to consider what is being proposed, what the potential implications of that action would be, and whether we, as a nation, wish to undertake the full consequences of that action.  The Constitution, rightly, does not allow a single person to commit the nation to war, even where there is a good reason to do so.  Where there is no imminent threat that does not allow for prior consideration, we must follow that process."

Update: A spokesman for Nadler emails to disagree with my characterization of the argument, saying, "this is not a squarely left versus right question, but rather one of congressional versus presidential authority. The power to declare war rests entirely with congress."