The enemy votes to declare war, not the Senate.
Maybe you should review the start of WWII: the attack, the Japanese declaration of war, the Senate's declaration of war at the Presiden't request. It was, in fact, while he was addressing the Senate that the famous lines we hear all the time about the 'Date which will live in infamy.'
He was before the Senate asking them to declare war on Japan. That is what the Constitution requires a President to do if he wants war powers and, by definition, if he wants the US to involve itself in a war. Otherwise, the definition, I believe, is a 'conflict.' The Vietnam war was, by legal definition, a conflict: the US never actually declared war on Vietnam, IIRC.
Gee...I wonder why there hasn't been a declaration of war...
Tuor