Two past Presidents could have faced prosecution, and now, Donald Trump faces the likelihood of prosecution when he leaves the Presidency.
John Tyler, (1841-1845), gave up his US citizenship when he declared his loyalty to the Confederate States of America in 1861 at the beginning of the Civil War. He became a member of the Provisional Confederate Congress in that year, voted for secession at the Virginia Secession Convention, and was elected to the Confederate House of Representatives, but died in February 1862 before the opening session of that legislative body.
Had he lived through the Civil War, Tyler might have faced treason charges, and be forced to face trial and possible conviction and imprisonment. As it was, the US government did not officially recognize his passing, and the flag did not fly at half staff, as it has for every Presidential death and funeral, except for Tyler. He was buried with a Confederate flag, rather than an American flag, over his casket
Richard Nixon (1969-1974) was the second President who could have faced prosecution and possible conviction and imprisonment as someone who had obstructed justice and abused power, and engaged in massive corruption in office. The fact of his resignation in August 1974 did not bar that possibility, but his successor, President Gerald Ford pardoned him from any prosecution in September 1974, a factor in Ford’s failure to be elected to a full term in 1976.
The obstruction of justice and abuse of power of Nixon has been revived due to the tumult surrounding the possible impeachment moves by Democrats in the House of Representatives against Donald Trump. By comparison, many observers see Nixon as not as evil as Trump.
Donald Trump will likely face prosecution when he leaves office in 2021, although if he wins a second term, he would likely prevent such action by the statute of limitations on prosecution. Of course, he could also be pardoned from any prosecution by a future President, so one has to wonder if Trump will be held accountable for Russian collusion, obstruction of justice, and abuse of power, crimes seen by his critics as far worse than anything Nixon or Tyler did in the Presidency.