It could be as simple as if he gets the nomination there is no issue.
If he does not AND he does run as a third party, do not vote for the third party.....unless you want the democratic candidate to win.

I suspect it ill not happen and if it does I would hope those in favor of Trump would understand the would be handing the election to the democratic candidate. Could/would they follow Trump to the extent of throwing the election? I would like to think not.