I haven't read the other replies, but here is my understanding.
The people who support him fall into 2 general categories.
The first is the easiest to explain. They think all American politicians are equally awful, equally criminal, and the only difference between them is who gets caught. Because of this, criminal convictions, and wrong doing by an American politician makes no impact on them. "They" are all the same. The details distinguishing any and all American policians are ignored. Why? It's the same circular reasoning I've been posting about recently. It's a real problem. Also, this group doesn't trust the media to give them accurate news about American politics
unless it is reporting the faults and failures. In politics, they are extreme pessimists. There's several reasons why this is emotionally and intellectually rewarding for them. For these reasons, they hate goverment, by default, and naturally vote republican because they are supposed to be the "small" government party, and hopefully they will lower taxes and be the lesser of two evils.
The second group is a little more complex to describe. This is a faith-based group. There's many factors at play which prop-up the convicted criminal as a hero who is being attacked by a "deep-state-conspiracy". Part of faith-based reasoning involves this idea of "compounding-increasing-probability". I put it in quotes because they have it entirely flip-flopped. It goes like this. Faith-based reasoning takes improbable loosley related explanations for phenomena and tries to link those improbable explanations together into a chain. They realize that each individual explanation on its own is unlikely, but they erroneously think that by linking up all of the highly improbable explanations they will "add-up" to something which "points-to" their fringe improbable theory. But, like I said, they have it backwards. Linking up 3 unlikely loosely related possibilities
reduces the likelyhood of the
chain as a whole. It does not increase it. They've got it backwards.
So, using this faith-based reasoning, the individual in the 2nd category starts collecting individual unlikely explanations for phemomena which reimagine current events such that Trump is actually the hero, not the villian. They're on the internet, they're consuming fringe and/or unreliable media, and they are collecting these unlikely explanations of events, and forming a chain in their mind which is imagined to producing an undeniable conclusion. In addition they are on social media where these fringe theories blossom, are reinforced, and those who believe in them are encouraged.
Once the conclusion is reached, they become a "Trumper". From here on out, any additional fringe theory about a "deep-state-conspiracy" is just one more link in the chain which they already erroneously think has added up to 100% certainty, when in fact it has plummeted, and is continuing to plummet rapidly into single digit territory, because, the longer the chain of these unlikley explanations of current events, the compounding is decreasing the likeyhood of the fringe theory, not increasing it.
This is why, at this point, any bad news that comes out against Trump in America is actually helping him. His supporters will take any bad publicity, craft an improbable flipped upside-down explanation for it, and add it to the list of all the others, magically reinforcing their alternate reality. These improbable explanations can easily be distributed and consumed by the "Trumpers" via social media. The response to anything bad that is reported about Trump can be rapidly squashed and flipped into an advantage overnight.
So that's it. You've got a coalition of extreme pessimists and extreme optimists in an unholy alliance supporting a criminal politician in America. And there's very little to be done about it.