Trump in 2016:
She shouldn’t be allowed to run.
If she were to win this election, it would create an unprecedented constitutional crisis.
In that situation we could very well have a sitting president under felony indictment and ultimately a criminal trial.
It would grind the government to a halt.
Source: https://www.independent.co.uk/tv/news/donald-trump-trial-guilty-hillary-clinton-b2556563.html
Same reason felons can vote; if being a criminal removes your ability to participate in the political process then the government suddenly has a very strong incentive to criminalize their political opponents.
TBF, if trump gets elected again, then the SC is going to legitimize political assassination, so being able to run from prison becomes a moot point.
Felons mostly can’t vote, though.