Justice Powell explained why the president's claim of unchecked power to spy within the nation's borders violates the Constitution:

Lord Mansfield's formulation touches the very heart of the Fourth Amendment directive: that, where practical, a governmental search and seizure should represent both the efforts of the officer to gather evidence of wrongful acts and the judgment of the magistrate that the collected evidence is sufficient to justify invasion of a citizen's private premises or conversation. Inherent in the concept of a warrant is its issuance by a "neutral and detached magistrate." Coolidge v. New Hampshire, supra, at 453; Katz v. United States, supra, at 356. The further requirement of "probable cause" instructs the magistrate that baseless searches shall not proceed.

These Fourth Amendment freedoms cannot properly be guaranteed if domestic security surveillances may be conducted solely within the discretion of the Executive Branch. The Fourth Amendment does not contemplate the executive officers of Government as neutral and disinterested magistrates. Their duty and responsibility are to enforce the laws, to investigate, and to prosecute. Katz v. United States, supra, at 359-360 (DOUGLAS, J., concurring). But those charged with this investigative and prosecutorial duty should not be the sole judges of when to utilize constitutionally sensitive means in pursuing their tasks. The historical judgment, which the Fourth Amendment accepts, is that unreviewed executive discretion may yield too readily to pressures to obtain incriminating evidence and overlook potential invasions of privacy and protected speech.

. . .

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.