If you know people who support Trump or have a significant chance of doing so in the future, I want to recommend a few brief videos you can send them. Trump's behavior in these videos is inexcusable and reflects poorly on his electability. If you know anybody who's a Trump supporter or has a good chance of becoming one, consider forwarding one or more of these videos to him.
Here's a recent one of Trump repeatedly acting as if he doesn't know who David Duke is and failing to condemn Duke and the KKK when asked to do so. Even if he's denounced such individuals and groups on other occasions and will do so again later, his incompetence in handling the interview is appalling. You want to nominate somebody that incompetent, who keeps getting himself into so much trouble, making himself and his party look so bad?
Here's one of Trump mocking a handicapped man. Yes, that's a quality you'd want in any individual, especially your presidential candidate in a campaign as important as this one. I know that Trump claimed that he wasn't intending to imitate the man's handicap, but the idea that Trump's gestures just happened to look like an imitation of the man's handicap is ridiculous. You'd have to be guzzling a lot of Trump Kool-Aid to buy Trump's explanation.
Here's Trump claiming that judges sign bills. On his radio program, Hugh Hewitt made a comment to the effect that Trump's remarks reflect a knowledge of civics below an eighth grade level.
Here's a video critiquing Trump's comments on the nuclear triad.
Here's Trump on forgiveness and communion.
There are many more videos like these available. These are just five examples of how ignorant, unethical, and unelectable Trump is.