What should be done if the President is mentally ill?

If!

Well, it's a hard question to answer. For one, he isn't the only president to be mentally ill in some capacity, so that alone can't be the metric we run with. What we do have is a poor politician and a weak leader who isn't really capable of handing the responsibility given to him. More on, there are people who have invested so much into the idea of what he is that they refuse to see the reality of what he actually is and what he has actually done.

When people believe someone who rarely tells the truth and think it's the rest of the world that is crazy, you're left with a special kind of echochamber that is incredibly hard to crack.

There are people who believe it is more plausible to have a secret global conspiracy aligned to bruise Trump's ego, over the reality of Trump simply being thin skinned and having little real experience or idea of what he is doing. Trump isn't really the root of the problem, just a symptom of an illness that is becoming terminal.

If anyone who has power in the government is mentally ill, I suggest they are placed under mental health care and depending on the severity, to be temporarily replaced until they get better. If they never get better then that means they need to leave anyway to work on themselves

Reagan was evaluated with Alzheimer's before his presidency had expired! Oh you did not know that though!

A new Presidential not Party Special Election,between Joe Biden,and Paul Ryan,2 Be held in May with 4 key debates.No extension of term.

Off topic... @ssg mark colwell
I don't know you... and I sure don't know why you've blocked my commo with you. If I said something that you were offended by, at least let me know what it was... and check out Matthew 18:15. Is that fair enough?

There are processes in place that cover that scenario as well as misconduct and almost everything in between. We're not there yet and only time will tell if we ever are, so it's all conjecture at this time. Let them entertain us with all the hearings, investigations and spin for now, and if events lead to remedies we'll find out at that time. For now just do damage control at the personal level, try to stay informed, and enjoy the ride as best we can.

As to his sanity roughly 63 million voters evaluated his fitness for duty and voted accordingly. Roughly 3 million more disagreed, but that's an assumption because they may have voted against him for other reasons. If the fear is that he will decide to nuke a country because he disagrees with a media story or tweet, I'd guess there are checks and balances to prevent that, hopefully. Considering the source, NYT recently reported that the intel community is routinely withholding information from him out of fear that he will share it with our enemies, I have to consider the source. That story was refuted by some representing said agencies. Credibility lost is hard to regain and there is plenty of blame outside of the WH on that point.

Maximus: I knew a man who once said, "Death smiles at us all. All a man can do is smile back."

To keep within the spirit of your question, remember that the USA is not a monarchy or dictatorship. No matter how good or bad the president is, they only have 33.3333% of the credit/blame. So as the president does not have 100% of the power, is there really an issue?

And I'm asking fairly. After all, do mentally ill senators/congressfolk get "retired" or forced out?

Trump def needs replacing. He is a the biggest security threat our nation has suffered since...since.... Klaus Fuchs comes to mind.