The constitution clearly says that a declaration of war is needed, and only short periods of action that are necessary in the interim can be taken by the president absent that. Congress does not have the authority to simply pass a law invalidating that. They did it, but it is most certainly not constitutional.

Then the question is whether the law passed by Congress was unconstitutional, which would have nothing to do with Obama. First someone with standing would have to challenge the law and then it would make its way up through the courts. But I'm pretty sure the Supreme Court would uphold it (as much as I would want it struck down along with the Patriot Act and other "war on terror" measures).

> Congress does not have the authority to simply pass a law invalidating that. They did it, but it is most certainly not constitutional.

The ones that should judge if it's not constitutional just passed judgment stating that a ex-president is above the law. I think they can invalidate if a president explicitly needs declaration of war, given recent history I'd bet a lot they would find it totally constitutional.