No, a U.S. President cannot pardon U.S. citizens who have not been convicted of a crime. The power to grant pardons, as outlined in Article II, Section 2 of the U.S. Constitution, is specifically for "Offenses against the United States," which implies that there must be an actual conviction or at least an offense for which one could be legally charged. This interpretation has been upheld by legal precedents, which emphasize that a pardon is an act of forgiveness for a crime already committed or an offense adjudicated. Therefore, without a conviction or at least an indictment, there is no legal basis for a pardon.