Would that have been true in the case of Ford's pardon of Nixon?
Or is this a matter of assuming Nixon did whatever he did that may have been indictable?
The House had already voted to impeach Nixon, which is the political remedy. Nixon avoided that by resigning from office. But Nixon's actions in covering up and interfering with the investigation and prosecution of a criminal action (the Watergate Hotel break-in) could have made him an accessory to it and separately criminally liable for obstruction of justice. This was, of course, before the Supreme Court lately granted all but monarchical immunity to the President. Therefore Nixon had every right to fear being prosecuted criminally for those activities.
On Sept. 8, 1974, then-President Gerald Ford pardoned Nixon using this language :—
As a result of certain acts or omissions occurring before his resignation from the Office of President, Richard Nixon has become liable to possible indictment and trial for offenses against the United States. Whether or not he shall be so prosecuted depends on findings of the appropriate grand jury and on the discretion of the authorized prosecutor.
* * *
Now, Therefore, I, Gerald R. Ford, President of the United States, pursuant to the pardon power conferred upon me by Article II, Section 2, of the Constitution, have granted and by these presents do grant a full, free, and absolute pardon unto Richard Nixon for all offenses against the United States which he, Richard Nixon, has committed or may have committed or taken part in during the period from January 20, 1969 through August 9, 1974.
(emphasis in original) Ford laid out his belief that Nixon was about to face criminal indictment and pardoned him for any crimes he may have committed during a period of time you might recognize as corresponding to his term in office. All the contemplated actions occurred before the pardon was issued, so the pardon is not prospective. The fact that the President doesn't even need to know if any crimes were actually committed and can just issue a blanket pardon is understandably controversial.