Answer:
The answer depends on your perspective. In Nixon, the US Supreme Court used judicial review to check the power of the President, limiting the Chief Executive's constitutional right to invoke Executive Privilege. Critics of the Court's decision may say that the Supreme Court overreached its authority by exercising judicial review (which is an implied power) and by placing restrictions on the President's use of Executive Privilege, which some may consider absolute. From that perspective, US v. Nixon, (1974) may be considered an instance of judicial activism.
The Supreme Court faced a dilemma in Nixon, in that allowing the President to withhold evidence under the doctrine of Executive Privilege abrogated Fifth and Sixth Amendment rights of the defendants charged with participating in the Watergate break-in and its cover-up.
In response to an argument that Special Counsel Leon Jaworski didn't have authority to subpoena the tapes, the Court held that 28 USC ยง 503 made the Attorney General head of the Department of Justice, charged with investigating and prosecuting crimes. Congress provided that the AG could delegate authority to the Special Prosecutor, and that Nixon could not rescind this power.
The President is not above the law. In the Watergate case, Nixon's recordings of conversations about the Watergate scandal essentially made him an unindicted co-conspirator. The President cannot (is not supposed to) use the power of his position to protect himself against criminal charges.
Proponents of the Court's decision may consider their constitutional interpretation valid, and not a case of judicial activism, although it probably wouldn't really be considered judicial restraint, either.
Chief Justice Warren Burger wrote, in the opinion of the Court:
"However, neither the doctrine of separation of powers nor the need for confidentiality of high-level communications, without more, can sustain an absolute, unqualified Presidential privilege of immunity from judicial process under all circumstances. The President's need for complete candor and objectivity from advisers calls for great deference from the courts. However, when the privilege depends solely on the broad, undifferentiated claim of public interest in the confidentiality of such conversations, a confrontation with other values arises. Absent a claim of need to protect military, diplomatic, or sensitive national security secrets, we find it difficult to accept the argument that even the very important interest in confidentiality of Presidential communications is significantly diminished by production of such material for in camera inspection with all the protection that a district court will be obliged to provide."
Case Citation:
United States v. Nixon, 418 US 683 (1974)
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