Simply ask someone with TDS: "What exactly has Trump done that you did not like?", "What proof exists which shows he's done X?". Each time you can watch them squirm as they realize that the thoughts they have are not their own and so they have no ability to speak in support of them.
One twitter poster is not much. And ask any republican why they hate any Democrat and their words are just echos of Trump and his campaign.
But let me answer the question:
Tax Fraud via his Charity, if found guilty.
Personally, my biggest issue is that he is an asshole. He is arrogant, constantly praising himself for actions, stating he knows more than he does, and attacks anyone (literally anyone) who disagrees with him even respectfully. Remember Jeff Sessions? Once Jeff disagreed on how to proceed with the russian probe (by excluding himself) Trump went from 'Jeff is the best' to 'Jeff is the worst' immediately.
He has fired people VIA TWITTER, which is just horrible for any boss to do. And not "announcing it via twitter to the public after telling the person" but literally tweeting that X is being replaced and having X find out from the press.
He threw his own intelligence agency under the bus in favor of Putin's word.
He constantly fights the press, especially when they ask him to explain his outlandish claims like how he had the biggest innauguration crowd ever.
He had his doctor read a statement stating that he is the healthiest person to ever be elected president.
He ended NAFTA just so he can rename it and claim it as his own. (He could have just renegotiated changes under the same name)
He wanted his fucking signature on a check that he isn't paying so he could get maximum credit. I don't recall Obama wanting his name on any stimulus checks.
If Mike Pence was president and had the same policies, he'd be attacked far less than Trump.