If congress passes any legislation that attempts to limit presidential powers, isn't that unconstitutional? One branch can only "impede" another where the constitution expressly permits--no?
The argument that the President is raising is that the authority for warrantless electronic surveillance comes from a combination of inherent powers of the executive (to protect the country against invaders) and the Congressional grant of power to use military force against the terrorist organization(s) who perpetrated the attacks of 9/11.
At the same time, the President is arguing that the FISA statute can be construed in such a way that the authorized action is not afoul of FISA - that Congress when it passed FISA meant to and did "carve out" circumstances such as "inherent power plus AUMF."
With regard to a conclusion of "unconstitutional," that term is really loaded and means different things depending on context. Permit me to use an example of a President encroaching on the legislature (and on the people) by raising Lincoln's suspension of access to courts (suspension of habeas corpus) during the Civil War.
No question he did it, suspended habeas corpus. And no question that as the executive (he has the guns) there is no superior force to cause him to relent "quickly." A "not quickly" relent would be to lose the next election or to be impeached, but that's a digression. After the fact, the Supreme Court reviewed the cases, and concluded that the Presidents suspension of habeas was in one case (Merryman) unconstitutional because it was unilateral, and in another case (Milligan) was unconstitutional because the denial exceeded the suspension criteria in the Congressional grant.
And that gets me to your "where the constitution expressly permits." The Constitution isn't detailed - it's "expressions" are mostly of principle, and generally lack detail. Past circumstances disputed, negotiated and recorded in court cases, the Congressional Record, and Presidential writings provide the details. And it will be perpetually argued (as long as the republic exists) that a particular action is unconstitutional, or constitutional.
In the case of the NSA issue, not only are Congress and the president butting heads, a right of the people embodied in the 4th Amendment is also in the constitutional/unconstitutional balance. And one should ask "who decides whether the search is 'reasonable'?" The Constitution in general says that NO branch can decide an issue unilaterally. In the case of executive action, Congress giving authority and the Courts concurring in the judgment. The very presence of these checks and balances is what gives the people some control over their government.