The most relevant precedent is United States v. United States District Court (Keith). Decided in 1972, Kieth involved a prosecution for conspiracy to blow-up a CIA office. The Executive argued that in order “to gather intelligence information” that was “necessary to protect the nation from attempts . . . to attack and subvert the existing structure of the Government,” it was constitutionally entitled to engage in electronic surveillance of American citizens without complying with the requirements of the Fourth Amendment. In Kieth, the Supreme Court unanimously and unequivocally held that, even in national security investigations, the President had no constitutional authority to conduct electronic surveillance of American citizens on American soil without a judicially issued search warrant based on a finding of probable cause.

In reaching this decision, the Court carefully considered and emphatically rejected the Executive’s demand for an exemption in national security investigations from the ordinary requirements of the Fourth Amendment.

Stone points out that some supporters of Bush's policy dwell on the fact that the Keith decision refused to address surveillance involving foreign powers or agents because that precise issue was not before it. He writes,

Although the reasoning of Keith would seem to apply to foreign as well as domestic threats to the national security, at least insofar as the surveillance involves wiretapping American citizens within the United States, Keith left the question unresolved. Thus, it is possible to argue, even after Keith, that Bush’s spy program, which purportedly is directed at those who communicate with foreign-based terrorists or terrorist organizations, is not necessarily prohibited by established Supreme Court precedent.

That argument by Bush supporters rings hollow, Stone says:

At the same time, however, it is important to emphasize that no court has ever held that the Executive can engage in electronic surveillance of American citizens on American soil in the context of “foreign intelligence” investigations without complying with the Fourth Amendment. Defenders of the Bush program have trotted out phrases from lower court opinions that have merely assumed arguendo that such searches might be permissible, but none has ever even addressed the question. Indeed, in the opinion cited most often by the defenders of the Bush spy program, In re: Sealed Case, the electronic surveillance at issue was based upon both a warrant and probable cause. The “precedent” relied upon by the administration’s defenders is not “precedent” at all.

The Court in Kieth left open the precise application of the Fourth Amendment to foreign intelligence activities, but the logic of the opinion leaves little doubt that the Bush spy program violates the Fourth Amendment.

This is an excellent, scholarly but understandable read and I highly recommend it to all.