THE U.S. COURT of Appeals here has rejected former president Nixon's argument that every president is completely immune from damage suits based on his official acts. This somewhat esoteric ruling grows out of the wiretaps the Nixon White House 10 years ago put on several government officials and newsmen. While other proceedings in which the taps figured - the Ellsberg case and the impeachment hearings - have been more spectacular, this decision actually imposes sharper limits on the power of the presidency than they did.

In rejecting the expansive view of presidential power always asserted by Mr. Nixon, the court said the same rules apply to presidents as to Cabinet officers and other high officials, with one possible exception. The rules make government officials liable for damaging someone's constitutional rights by an illegal act unless they can show they had reason to believe that what they were doing was legal and that they acted without malice or bad faith.

These qualifications may be sufficient to spare Mr. Nixon and others who were involved in those wiretaps having to pay damages. They can argue that in 1969 it was not clear the president lacked authority to order wiretaps. Mr. Nixon had been told he had that power by Attorney General Mitchell and FBI Director Hoover before the taps were placed on the phones of Morton Halperin, then a member of the White House staff, and Hedrick Smith of The New York Times.

It has been clear all along that such taps were illegal unless they had something to do with protecting the national security. That is the gray area in which the court said it may be possible for the president to commit otherwise illegal acts without being held liable.

But the judges defined national security quite differently from the way Mr. Nixon did. They ruled that if a president's special obligation to protect the nation does give him special powers, these can be exercised only in instances of "immediate and grave peril" to the nation. An investigation of who was leaking information to the press is not such an instance, they said.

This narrow view of national security, coupled with the new personal liability of presidents for illegal acts, could be regarded as a direct judicial challenge to presidential authority. It is, therefore, a likely candidate for review by the Supreme Court and a reopening of the debate on what a president's national-security powers really are.