This leads to an interesting thought experiment - What would happen if Congress declares a war the President didn't ask for? A Declaration of War requires only a simple majority to pass. It is a resolution, not a law. As such a Declaration of War is not subject to a presidential veto.
Obviously, President Obama would refuse to prosecute the war. But, Iran may not believe his diplomatic statements. They could respond by mining the Strait of Hormuz, trapping much of the Fifth Fleet (up to two carriers and 20 ships) in the Persian Gulf where it can be attacked relentlessly. Israel would do nothing since the whole point of this is to force the US to do the fighting for her. There would be no grand global alliance against Iran as there was in the Afghan and Iraq wars. The United States would be forced to fight a major war alone for the first time since 1898. And the war would be popular in some places as sharply rising oil prices would be a boon to US oil producing states.
All of this is wildly unlikely. It's more likely that Mitch McConnell is an actual turtle-human hybrid. It's also possible that Netanyahu's speech will blow up at home and he will lose the Knesset elections set for two weeks from now. We can only pray.