From the NYT:
Nor does the N.S.A. program conflict, the Justice Department said, with what many legal analysts had regarded as the exclusive authority for intelligence wiretaps under the Foreign Intelligence Surveillance Act, passed by Congress in 1978 in response to Watergate-era political abuses. Some presidential powers, particularly in the area of national security, are simply "beyond Congress’ ability to regulate," it said.
From the United States Constitution (Art. I, Sec 8):
The Congress shall have power…
To declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water;
To raise and support armies, but no appropriation of money to that use shall be for a longer term than two years;
To provide and maintain a navy;
To make rules for the government and regulation of the land and naval forces;
To provide for calling forth the militia to execute the laws of the union, suppress insurrections and repel invasions;
To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof.
Especially when you take into account this final paragraph, it’s quite obvious that the Constitution empowers Congress to regulate quite heavily — almost universally — in the area of national security.
Glenn Greenwald has much more to say about the Administration’s "naked theory of limitless presidential power" in times of war.