What does the US Constitution require of him? Only these things:
- That he faithfully execute his office (per his oath);
- That he preserve, protect and defend our constitution (per his oath);
- That he faithfully execute our laws;
- That he be Commander-in-Chief of the armed forces; and
- Other miscellaneous duties specified in Article II.
As to other standards of "fitness" which some may wish to apply, I can only say this: the Founders established the Electoral College for the very reasons that they did not believe political parties or a popular vote could be relied upon to select the best person for the job. The College has been so tampered with over the years that it no longer serves that purpose. In effect, it is Congress which has allowed this situation to develop, and it is Congress which will have to fix it.
But don't expect them to. They would rather flush the whole system down the toilet and start over. Their continual whining about Trump seems designed to facilitate that very process.
And this makes them, not the President, the greatest disrespecter of the constitution in the land.
I weep for our country. It won't be around in this form for much longer.