from the Washington Post:
In
Ex parte Garland , the Supreme Court settled the question of preemptive pardons. The justices in that 1866 case decided that while pardons could reach only past acts, the pardon “may be exercised at any time after [the act’s] commission, either before legal proceedings are taken or during their pendency or after conviction and judgment.”
Even before
Garland, President Abraham Lincoln (among others) pardoned dozens of people — including alleged traitors — preemptively. More recently, President Jimmy Carter pardoned hundreds of thousands of Vietnam draft evaders, including those who had not been charged or convicted. And, most famously, President Gerald Ford pardoned President Richard Nixon, who had not yet been charged with anything.