State charges, currently. There's actually no law on the books keeping someone convicted of a federal crime from running for president, to the best of my knowledge (not a lawyer). Conviction via an impeachment trial is the only way.
It would also be rather difficult to run the country from inside a jail cell, though. Or even a campaign. He wouldn't be allowed to show up for debates, tour the country giving speeches... things we just take for granted that candidates are able to do.
Frankly it'd be better if he did run, and spare us the possibility that some other Republican could win.
The GOP will absolutely let him take the primary again, because the cult follows trump and they cannot win the election without the cult.
They cannot win the election with trump in at all. Even with the greatest possible clone, the majority of the population does not support the culture war bullshit and complete lack of policy. They have only gotten as far as they have because of how stupid the gerrymandering and voter roll purges have gotten.
I really don't think he's going to be getting the nomination or even be the last contender. I think his star is falling and several other people are able to capture his fire.
A few months ago I could have seen that, but Trump was gaining momentum even before all this. Desantis waited too long to announce (and still hasn't), and the other candidates are, um... who? The GOP field is very boring so far.