The Constitution says:
U.S. Constitution - Article 3 Section 3


“"Treason against the United States shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open Court.”

We The People should have the right to demand that any sitting President be held accountable for their words and action if they give the appearance that they are not of sound mind, mental state, or judgement. 
The lack of political experience is no excuse for lying. Nor should one political party be allowed to block or interfere in any investigation of the White House. They are not by default above the law.

It is clear that Donald Trump is not fit for the office of President, but the proper protocols must be followed. The United States of America's Constitution must be followed and it is clearly not. So-called President Trump does not even understand the Constitution. 
  
Besides what might amount to high crimes and misdemeanors , there are all of the bigotry, lies, falsehoods, misrepresentations, broken promises, smoke screens and plain old stupidity displayed on a daily basis by Trump and his crew of misfits.

Article II (Article 2 - Executive) Section 4

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.

Amendment XXV  affects 9 (Amendment 25 - Presidential Vacancy, Disability, and Inability)

4: Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.


Thereafter, when the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that no inability exists, he shall resume the powers and duties of his office unless the Vice President and a majority of either the principal officers of the executive department or of such other body as Congress may by law provide, transmit within four days to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office. Thereupon Congress shall decide the issue, assembling within forty-eight hours for that purpose if not in session. If the Congress, within twenty-one days after receipt of the latter written declaration, or, if Congress is not in session, within twenty-one days after Congress is required to assemble, determines by two-thirds vote of both Houses that the President is unable to discharge the powers and duties of his office, the Vice President shall continue to discharge the same as Acting President; otherwise, the President shall resume the powers and duties of his office. {Passed by Congress July 6, 1965. Ratified February 10, 1967.}