That's the question David Safier (TucsonWeekly/The Range) would address, not to Trump, but to the leaders of the Republican party.
But I would love to ask every Republican with power and influence in the party — every elected official and every unelected member in the top ranks of the party — have you no sense of decency? How can you continue to further Trump's candidacy, either with outright endorsements or by criticizing specific things he says while continuing to support him? Do you really believe this man, who deserves to have his vast collection of emotional and psychological weaknesses written up in a psychiatric journal, is fit to be President of the United States and Commander in Chief of the largest and most powerful armed force in the world?
I have no doubt leaders of the Republican Party have bitten their nails to the quick and are watching for Trump's next outrage with knotted stomachs. I'm sure as I write this, they're asking themselves, "How far is too far?" The clear answer is, Trump has been going too far for far too long, while too many Republicans have held their noses and tried to tolerate his outrages for the sake of the party. If they have any sense of decency, if they put country above party, they must do everything in their power to make sure Trump is not elected president, and try to salvage what's left of their diminishing self respect before it's too late.
A Preemptive False-Equivalency Note: To anyone who plans to write an "Oh yeah, what about Hillary?" comment: No, it's not the same thing. Trump's unfitness for office is a thing unto itself, unlike anything I've seen in a presidential candidate of a major party in my lifetime.
But why not ask Trump?
Actually, I wouldn't bother asking Trump, "Have you no sense of decency?" No point. He'd just blather on about how he's probably the most decent man you've ever met, how his decency built buildings and hired workers and helped make America great.