I really do think congress has to push for an urgent evaluation.
His health evaluations, by the doctors he chooses, say what he wants them to say: Need I remind you:
"His physical strength and stamina are extraordinary, If elected, Mr. Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency."
Not a single republican felt that Trump's solicitation of bribery of a foreign power to announce a sham investigation into his opponent was impeachable.
But for arguments sake: let's say that the house and (republican controlled) senate manage to demand one.
He won't do it. This is a whitehouse that simply refuses subpoenas (from congress, from the pentagon, from special investigators).
But let's imagine that congress demands it.
Let's pretend that he actually takes it.
Let's assume it shows him as mentally unfit.
Let's imagine that the result gets out.
He not only won't be removed (that would require his hand-picked cabinet to agree), He not only won't be impeached (that would require some GOP support)... it won't even hurt his chances at re-election. His supporters will see it as a conspiracy or, as they have done with his admitted sexual assault, his admitted child porn (is that the correct term for deliberately walking in to where underage beauty contestants are naked?), his numerous affairs and divorces, his mocking of the handicapped, his racism, his draft dodging, his treason... they will not care because <insert reason> and vote for him anyway. It's part of their identity at this point.
Oh sure. Some will claim they are "independent" and they don't like him... and then proceed to rationalize why the focus should be on not voting for his opponent ("but her emails").
But it's not like this mental inability is new. He's been doing this for years (
https://www.amazon.com/Dangerous-Case-Donald-Trump-Psychiatrists/dp/1250179459)