The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.
Vice-President Biden’s old position on presidential war-making power:
The President has no authority to use force in Iran unless Iran attacks the United States, or there is an imminent threat of such an attack. The Constitution is clear: except in response to an attack or the imminent threat of attack, only Congress may authorize war and the use of force.
I want to make it clear. And I made it clear to the President that if he takes this nation to war in Iran without Congressional approval, I will make it my business to impeach him. That’s a fact. That is a fact.
Secretary of State Hillary Clinton’s old position on presidential war-making power:
If the country is under truly imminent threat of attack, of course the President must take appropriate action to defend us. At the same time, the Constitution requires Congress to authorize war. I do not believe that the President can take military action – including any kind of strategic bombing – against Iran without congressional authorization.
We are now at war without any imminent threat of attack, and without any Congressional consultation, much less approval, which makes one thing completely clear: This bunch didn’t mean one word of what they said. They are completely full of it. As Joe Biden would say, that is a fact.
UPDATE: Biden is getting grief for his pledge to impeach the president if he takes us to war without Congressional approval. Plus, a second occasion on which he made the pledge.