It does not absolve him of his duty to uphold the constitution. In the Supreme Court case that examined this during the Vietnam era, there was a unanimous decision that even for matters of national securty, the president is still required to seek court approval before spying on US citizens. Here's a quote from the decision, followed by a link to the full decision:
"These contentions in behalf of a complete exemption from the warrant requirement, when urged on behalf of the President and the national security in its domestic implications, merit the most careful consideration. We certainly do not reject them lightly, especially at a time of worldwide ferment and when civil disorders in this country are more prevalent than in the less turbulent [407 U.S. 297, 320] periods of our history. There is, no doubt, pragmatic force to the Government's position.
But we do not think a case has been made for the requested departure from Fourth Amendment standards. The circumstances described do not justify complete exemption of domestic security surveillance from prior judicial scrutiny. Official surveillance, whether its purpose be criminal investigation or ongoing intelligence gathering, risks infringement of constitutionally protected privacy of speech. Security surveillances are especially sensitive because of the inherent vagueness of the domestic security concept, the necessarily broad and continuing nature of intelligence gathering, and the temptation to utilize such surveillances to oversee political dissent.
We recognize, as we have before, the constitutional basis of the President's domestic security role, but we think it must be exercised in a manner compatible with the Fourth Amendment. In this case we hold that this requires an appropriate prior warrant procedure.
We cannot accept the Government's argument that internal security matters are too subtle and complex for judicial evaluation. Courts regularly deal with the most difficult issues of our society. There is no reason to believe that federal judges will be insensitive to or uncomprehending of the issues involved in domestic security cases. Certainly courts can recognize that domestic security surveillance involves different considerations from the surveillance of "ordinary crime." If the threat is too subtle or complex for our senior law enforcement officers to convey its significance to a court, one may question whether there is probable cause for surveillance.
Nor do we believe prior judicial approval will fracture the secrecy essential to official intelligence gathering. The investigation of criminal activity has long [407 U.S. 297, 321] involved imparting sensitive information to judicial officers who have respected the confidentialities involved. Judges may be counted upon to be especially conscious of security requirements in national security cases. Title III of the Omnibus Crime Control and Safe Streets Act already has imposed this responsibility on the judiciary in connection with such crimes as espionage, sabotage, and treason, 2516 (1) (a) and (c), each of which may involve domestic as well as foreign security threats. Moreover, a warrant application involves no public or adversary proceedings: it is an ex parte request before a magistrate or judge. Whatever security dangers clerical and secretarial personnel may pose can be minimized by proper administrative measures, possibly to the point of allowing the Government itself to provide the necessary clerical assistance.
Thus, we conclude that the Government's concerns do not justify departure in this case from the customary Fourth Amendment requirement of judicial approval prior to initiation of a search or surveillance. Although some added burden will be imposed upon the Attorney General, this inconvenience is justified in a free society to protect constitutional values. Nor do we think the Government's domestic surveillance powers will be impaired to any significant degree.
A prior warrant establishes presumptive validity of the surveillance and will minimize the burden of justification in post-surveillance judicial review. By no means of least importance will be the reassurance of the public generally that indiscriminate wiretapping and bugging of law-abiding citizens cannot occur."
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