There are two ways that the American Constitution provides for the removal from office of a duly elected, living President.
Amendment XXV of the Constitution was ratified about 50 years ago and cleans up a couple of lingering questions including the provision of a path for succession should a living President be unable to fulfill the duties of the office through illness or other circumstance. Although there are those who would declare Trump unfit under Amendment XXV, that's really not a serious possibility.
Impeachment, however, is a serious possibility these days. Written into the body of the Constitution is the following:
The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
Note the distinction between impeachment and conviction. Impeachment roughly corresponds to an indictment. The House of Representatives impeaches (indicts) through a majority vote and supplies the managers (prosecutors). The President (defendant) chooses his/her own counsel, not necessarily elected politicians. The Senate acts as the jury. The Chief Justice of the Supreme Court presides. A two-thirds vote of the Senators present is required for conviction. The only penalty for conviction is removal from office.
Note the listed causes for impeachment. Treason and bribery speak for themselves. But what is the definition of a high crime or misdemeanor? In simple, real world terms, the answer to that question is that high crimes or misdemeanors are whatever the House says that they are. The history of Presidential impeachments is instructive in this regard.
Two Presidents have been impeached. Democrat Andrew Jackson was impeached by a Republican Congress in an egregious display of partisanship, primarily because of Jackson's refusal to adhere to a law, later deemed unconstitutional, designed to limit his Presidential power. He was acquitted by one vote on three of eleven articles of impeachment. Following the third acquittal, his trial was adjourned. Democrat Bill Clinton was impeached by a Republican Congress for lying under oath concerning his sexual activity with an intern and for attempting to obstruct the investigation. Neither of the two articles of impeachment managed to gain a clear majority of Senators, much less two-thirds.
It would not be a stretch to say that both impeachments were politically motivated and not based on circumstances that the Founders had in mind. But they were, nonetheless, legitimate. They followed form. There's a saying in American jurisprudence: A good prosecutor can convince a grand jury to indict a ham sandwich. Although a President is not a ham sandwich, the point is valid when it comes to impeachment. The House, sitting as a grand jury, can indict for whatever reason that it chooses.
Contrary to popular belief, Richard Nixon was not impeached, much less convicted. After the House Judiciary Committee passed articles of impeachment, it was clear that the House as a whole would vote to impeach, But it was not certain initially that the Senate would convict. After release of a particularly incriminating recording and after the Saturday Night Massacre (when Nixon fired high-level Justice Department officials who would not order an end to investigations), conviction in the Senate became a virtual certainty. Nixon resigned.
Can Trump be impeached? Certainly. Will he be impeached? Right now, that's an open question. I personally believe that he is most likely to be impeached if he continues to order that House subpoenas for testimony and documents be ignored. If that continues after a Supreme Court ruling against him, he most definitely will be impeached. If impeached, would he be convicted? Under that circumstance, defiance of a Supreme Court ruling, I have to believe that he will be. But we're not there yet. Like Speaker of the House Pelosi, my inclination is to go slowly and see if Trump obliges by putting the noose around his own neck.
