Some US presidents have used this regal power of leniency in a pointed way near the end of their term in office.

On the last day of his term in 2001, Democratic president Bill Clinton granted pardon in a highly controversial move to late fugitive trader Marc Rich, whose ex-wife had been a major donor to Democrats.

Sixteen years later, Obama is fielding pressure from all sides to grant unlikely pardons or commutations of sentences to people whose supporters say have been unjustly sentenced or sought out by the justice system.

Among them is Bowe Bergdahl, a US Army sergeant held captive for five years by the Taliban before his release in a prisoner swap, who is due to be court-martialed for desertion.

Leonard Peltier, a Native American activist convicted for the 1975 deaths of two FBI agents in what his supporters say was a setup, is also hoping to enjoy Obama's good graces.