Are people afraid to give an actual answer here? Just name things you don't like and if Trump does it then you stop supporting him. It's that simple and applies to every single person and anyone they politically support.
I have already done this on the first page, but very well, let's give a more detailed answer again for those late to the party.
If Trump did this I'd stop supporting him
1.
I am very concerned about War. One thing that I liked about Trump was that he did not get us into any new wars. It had been a long time since we had a president who didn't get us into new wars. That doesn't mean I don't support the army and the need to defend ourself, but I want it done legally. If Trump were to go and blow up a Russian pipeline, or try to assassinate Putin with Drones, or send F16 to Ukraine which is not an ally after promising not to, or if he sent US soldiers to fight Russia without a formal declaration of war, then yes, these would be things that would sour me on him very quickly.
2. I am very concerned about corruption in Washington DC. If Trump were to take my tax dollars to pay off college loans, I would consider that theft. If I found out that ten million dollars was paid to Trump through Don Jr. from a foreign country in exchange for Trump doing them favors, if I discovered that Trump's kids were able to get away with not paying taxes on millions of dollars because the IRS and FBI were protecting his kids, then yes, I would not support him.
3. I am very concerned about the weaponization of the DOJ. Just because I don't agree with someone does not mean I want their views censored. If I found out the DOJ was labeling parents as Domestic Terrorists because they voice complaints at school board meetings, if I found out that Trump was arresting his political enemies, if I found out that his campaign was using dirty tricks to falsely label his opponent as being in the pocket of Russia, or creating false flag events to discredit his political opponent (kidnapping a governor or having an insurrection) then I would not support him.
4. I am very concerned about child trafficking for sex. If Trump opened wide the border to let the child traffickers in, if child trafficking became the most profitable business for the cartels, even more profitable than drugs under Trump, if Trump himself were a pedophile taking showers with 11 year old girls and groping children in public, then of course like any decent person, I would not support him.
5. I am concerned about who he is as a person. Don't get me wrong, I don't expect our politicians to be saints, or even holy men of God. But I do expect them to be decent. If Trump did something despicable like refusing to acknowledge one of his grandkids, then I would not vote for him.
6. I am concerned that he be competent. If Trump could not put two sentences together, if he appeared to have dementia, diminished cognitive abilities, diminished physical abilities then I would not want him to be president.
7. I am also concerned about the economy. If I thought he handled the economy poorly then I would look for another more capable candidate.