"When the president does it, that means that it is not illegal." — Richard M. Nixon (1913-94)
Legal scholars have been fascinated for two centuries about whether an American president can break the law and remain immune from prosecution. During the Civil War, Abraham Lincoln ordered troops to arrest, without warrant, and incarcerate, without due process, many peaceful, law-abiding journalists and newspaper editors — and even a member of Congress — in the Northern states. Wasn't that kidnapping?
During World War I, Woodrow Wilson ordered federal agents to arrest people who sang German beer hall songs or read aloud from the Declaration of Independence in public. Wasn't that infringing upon the freedom of speech?
During the Great Depression, Franklin D. Roosevelt ordered banks to confiscate gold from Americans who had purchased and possessed it lawfully. Wasn't that theft?
In the early 1970s, Richard Nixon used the CIA to spy on Americans and to frustrate the FBI's efforts to investigate a burglary at the Democratic National Committee's Watergate headquarters, and then he denied doing any of this. Wasn't that an invasion of privacy and obstruction of justice and using a federal office for deception? Years after his resignation, Nixon infamously told David Frost in a live interview that no matter what the president does, it is lawful. Where did he get that argument from?
And President Donald Trump has asked Congress for money to condemn real estate and build a border barrier in Texas. Congress said no, yet he plans to spend the money anyway. Doesn't that violate his oath to uphold the Constitution?
Though there may have been political consequences to each of these presidential acts of lawlessness — there were for Nixon, at least — there were no legal consequences in the form of impeachment or prosecution. The Constitution itself limits impeachment to treason, bribery or other high crimes and misdemeanors.
The "high crimes and misdemeanors" language was interpreted by the House Judiciary Committee in 1974 to include material interference with a governmental function and obstruction of justice and the use of a governmental asset to deceive the public — but not any garden-variety crime, such as bank or tax fraud or kidnapping or invasion of privacy or misappropriation of federal funds.
During the presidency of Bill Clinton and afterward, the Department of Justice ordered research about whether a president could be charged with a crime against his will and while still in office. The DOJ now possesses three scholarly legal opinions on the subject. Two of them say he cannot be prosecuted; one of them says he can. All three are based on the same law and history but employ different deference to the presidency.
Two of those opinions say that if there is probable cause of crime by the president and the time during which the law requires a prosecution to commence — the statute of limitations — would expire while the president is in office, he or she should be indicted while in office but the prosecution should be deferred until he or she is no longer president. One of the opinions, incredibly, mimics the Nixonian president-as-prince idea and basically tells the Department of Justice to forgo prosecution.
Clinton was prosecuted while in office for testifying falsely — lying under oath in a civil deposition, a crime rarely prosecuted — but it was with his consent. Presumably, he consented to the quick prosecution and guilty plea in the final days of his presidency to avoid a costly and prison-exposed post-presidential indictment for more serious crimes.