In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President, and the Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected.
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.
These Constitutional clauses both allow for the impeachment and OR removal from office, of a President who is clearly acting against the best interests of the American People, and the United States.
The Republicans have already established the precedent that a President who lies to Congress can be impeached. Trump has told more lies than truths, so that's enough. However, I would myself prefer to look at the destructive incompetence he has repeatedly displayed while in office, endangering our military and our intelligence operatives, by openly revealing details of what we now and how we know it; ignoring and even praising attacks by other nations on our soil; that sort of thing.