It most certainly is because it is the liberals who are leading the charge in proclaiming Trump is delusional, not conservatives.
And you have to prove Trump is mentally ill.
Pffft!
Nobody has to "prove" anything about Trump's mental health. And of course Democrats are in the lead as far as talking about Trump's deficiencies; in case you missed it, Trump accomplished a hostile takeover of the GOP last year, and its members have dutifully put party before country ever since.
BUT - Republicans (other than those in 'safe' districts or who are not up for re-election in 2018) are already joining Democrats in calling out his incompetence, dishonesty and unfitness to lead. So far, only Dems (afaics) have pressed the fact that he is a danger to American democracy, but it's getting closer to the tipping point with every sequential Cheato fuckup. Won't be long now before we will know if he can endure until the mid-terms. Right now it looks like he might, and if he does and nothing has improved as far as his record-low approval ratings, and Republicans haven't turned on him, the house (and maybe even the Senate) will fall in 2018 and he'll be impeached before you can say "LOCK HIM UP!"
Of course if he manages to get the military to back him in the insidious coup he has undertaken, that could all be moot. In that case there will be rivers of blood in the streets - just like Uncle Vlad is hoping.