At Balkinization, Mark Graber: Taking the Text of the Twenty-Fifth Amendment Seriously. From the introduction:
Donald Trump is constitutionally unfit to be President of the United States. The Twenty-Fifth Amendment to the Constitution of the United States requires the Vice President to assume the presidency whenever “the President is unable to discharge the powers and duties of his office.” The Twenty-Fifth Amendment does not condition the temporary or permanent removal of the president on the president being “physically” or “medically” unable to discharge the powers and duties of his or her office. Rather, the Vice-President is to assume the powers of the presidency when, for any reason, "the President is unable to discharge the powers and duties of his office.” A president who is a congenital liar and a bigot, under the explicit words of the Twenty-Fifth Amendment is unable to discharge the powers and duties of his or her office.
The last paragraph of Section 4 supports claims that the Twenty-Fifth Amendment is not limited to physical or medical conditions. That paragraph authorizes Congress to resolve, by a two-thirds vote of both Houses, whether a president is able to discharge the powers and duties of the presidency. Members of Congress as a whole have no particular expertise on physical, medical, or psychiatric conditions. That is for medical professions, who are given no role in the constitutional process for removing the president. Members of Congress do have expertise on whether a president, for any reason, is capable of discharging his or her responsibilities. Thus, given that Congress cannot determine whether a president is a sociopath, but can determine whether the president is a congenital liar, the best reading of the Twenty-Fifth Amendment is that Congress should focus on whether the president is able to discharge the powers and duties of office and not on whether the reason for that failure is some physical or medical problem.
But as I have argued previously in response to a somewhat similar Balkinization post by Mark Tushnet, this position does not take the text seriously. In particular, it conflates "unfit" (in the first sentence of the post) with the constitutional word "unable." As I wrote earlier:
True, "unable" can have the figurative meaning "unfit" or "not able to perform effectively" in some contexts (he's "unable" to play quarterback because he throws a lot of interceptions). But it also (and I would say more commonly) means literally "not able" -- that is, not capable of doing the thing in question because of an absolute impediment (he's "unable" to play quarterback due to a broken leg).
And I would say that the history and structure of the amendment strongly confirm that the latter meaning is the correct one. In particular, if the Amendment meant that a President could be removed simply because he had character flaws that prevented him from governing in the way some people (or even most people) might prefer, that would be an extraordinary change in the constitutional structure of the presidency. (Perhaps it would be a good one, but it would not likely be one that went unnoticed for decades). Instead, it seems clear that the Amendment should be read to cover only to narrow circumstances when the President is literally unable to act as President. That is the reading that takes the text (and its structure and history) seriously.
Professor Graber's point about medical experts isn't to the contrary. The reason to give the ultimate responsibility to Congress is of course to place it with a politically accountable branch. Naturally Congress could consult with medical experts in reaching its conclusion. But ultimately the Constitution provides for us to be governed by representatives, not by experts, including in this matter.
Professor Graber continues:
Consider a president who goes on a permanent vacation and refuses to discharge any of the powers and duties of the office. For political purposes, no difference exists between that president and the brain-dead president. If we limit the Twenty-Fifth Amendment to presidents with physical or medical conditions, however, we can only remove the brain-dead president.
I agree that the Amendment is not limited to physical or mental conditions. The text has no such limit. Suppose the President is kidnapped by terrorists, or his plane crash-lands in remote wilderness. He is then "unable" to act as President, and the Amendment applies. But Professor Graber's example shows that he is not taking the text seriously. A President on permanent vacation is not "unable" to discharge his duties; he is "unwilling." The text says "unable," not "unwilling." Perhaps there is no difference "for political purposes" but there is a difference for textual purposes.
The remedy for the President on permanent vacation is impeachment. Refusing to discharge the powers and duties of the office is a misdemeanor (in the original sense: misconduct in office) for which impeachment would be appropriate. Similarly, character flaws that cause the President to take wrongful actions in office could lead to impeachment.
The structural key to the 25th Amendment is that it supplements the impeachment remedy for situations where impeachment isn't appropriate. A President who is "unable" to discharge the duties of office due to an external event not of his choosing (illness, injury, kidnapping, etc.) shouldn't be impeached, because he has not committed a "misdemeanor" (misconduct). Moreover, he doesn't necessarily need to be removed from office (the remedy upon impeachment and conviction) because the inability may be temporary. Thus the 25th Amendment sensibly provides a remedy that does not depend on misconduct and can be of limited duration. In contrast, if the President has character flaws that lead him to misconduct in office, as Professor Graber supposes, the remedy is impeachment; there is no need to try to stretch the 25th Amendment to cover something that is already covered elsewhere.