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.