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• depressive symptoms, such as loss of hope and thoughts of suicide or self-harm. New York Times reporters, Peter Baker and Maggie Haberman, recently described Trump as isolated in the White House ( https://www.nytimes.com/2018/12/22/us/politics/trump-two-years.html") The president has told associates he fells 'totally and completely abandoned" ... complaining that no one is on his side and that many around him have ulterior motives." On December 24th, Trump tweeted: "I am all alone (poor me) in the White House..."
• anxiety with high blood pressure, tense muscles, clammy hands, dizziness, upset stomach, and trembling or shaking. There have been rumors that Trump has high blood pressure; in addition his daily diet is terrible -- he prefers McDonalds ("a full McDonald’s dinner of two Big Macs, two Filet-O-Fish sandwiches, and a small chocolate shake") and 10-12 diet cokes -- and is at least 25 pounds overweight.
• insomnia. Trump's last physical (January 2018) indicated that he typically gets only 4-5 hours of sleep each night. (The typical person gets 7-8 hours each night.)
• hallucinations. There's no evidence that Trump has had classic hallucinations, such as seeing an extraterrestrial, but there's ample evidence that he tells tall tales that he believes. It's well established that Trump lies at an unprecedented rate. A recent Washington Post article (https://www.washingtonpost.com/politics/2018/11/02/president-trump-has-made-false-or-misleading-claims-over-days/? ) noted that Trump had made 6420 false statements over 649 days and in recent months had lied at the rate of 30 false claims each day -- with 84 false claims on October 1st. There's abundant evidence that Trump believes his most common falsehoods: the Washington Post compiled a list of Trump's repetitive lies ( https://www.washingtonpost.com/graphics/politics/fact-checker-most-repeated-disinformation/? ) such as the claim that the Trump tax cut was "the largest in history." Trump imagines these wild distortions to be true.
• extreme mood swings or unexplained outbursts. New York Times reporters, Peter Baker and Maggie Haberman, described Trump as, "a president who revels in sharp swings in direction, feels free to disregard historic allies and presides over near constant turmoil within his own team as he follows his own instincts." "When President Trump gets frustrated with advisers during meetings... he sits back in his chair, crosses his arms and scowls. Often he erupts, '[f***ing] idiots.'"

The 25th Amendment of the Constitution specifies the procedure to be followed if there is a disabled president: "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." (It's more complicated because the President can appeal; and this part of the process has never been used.) 

In the current case, Vice President Pence and as least eight of the fifteen cabinet secretaries would have to transmit to the Senate pro tempore -- who was Orrin Hatch (retired) and is now Chuck Grassley -- and the Speaker of the House -- likely Nancy Pelosi. 

To summarize, there are three ways for Trump to leave office before January 20, 2021. He can resign -- as Richard Nixon did (August 9, 1974); Trump can be impeached; and he can be removed due to mental (or physical) disability. 

What could push Republican leaders to declare Trump as mentally disabled? The most likely scenario involves Trump's abuse of his power as commander-in-chief. At the beginning of the Trump administration, there were three senior generals -- John Kelly, James Mattis, and H.R. McMaster -- that "moderated" Trump's notions about how to use the military. (For example, Trump wanted to assassinate Syrian ruler, Bashar Assad.) Now these generals are gone, replaced by less able men -- who have little or no military experience. 

Sadly, it's easy to imagine Trump doing something like sending the military to surround the U.S. Capitol building in an attempt to force the new Congress to appropriate money for his border wall. One can also envision Trump, when faced with impeachment proceedings, launching a reckless war in an effort to distract the nation. 

It's difficult to imagine Vice President Pence invoking the 25th Amendment, declaring Trump unfit for office, without the support of all of the leaders of the Republican Party -- donors as well as political leaders such as Mitch McConnell. But if Trump did something truly awful, we can foresee a conversation where GOP leaders confront Trump and say: "Donald, there's strong support for removing you from office either by impeachment or a declaration of disability. To avoid this, why don't you resign and 'President' Pence will grant you a full pardon."