Under Section 4 of the 25th Amendment, the vice president and a majority of cabinet officers or “of such other body as Congress may by law provide” may jointly decide that a president is unfit to serve. Following the 25th Amendment, on April 14th, Rep. Earl Blumenauer (D-Oregon) introduced the Strengthening and Clarifying the 25th Amendment Act of 2017, HR 2093. HR 2093 would “provide for an alternative body to transmit a written declaration that the President is unable to discharge the powers and duties of his office in accordance with the provisions of the 25th Amendment of the Constitution.” In a nutshell, if passed which it won’t in a Republican controlled Congress, HR 2093 would empower former presidents and vice presidents of both parties in coordination with the sitting vice president to determine if a president is fit for office. The motivation appears to be that the 25th Amendment would fall short in cases of emotional or mental incapacity because Cabinet officers might be too conflicted to make such a determination because they were appointed by the sitting president.
One can only wonder how the living former Republican presidents and vice presidents would vote on Trump’s fitness to lead the US. — Joe