And in case Trump IS nominated....
The two biggest problems: finding a well-known candidate willing to make the race, and beating the calendar. Time is running out for getting an independent or third-party candidate on the ballot.
If the effort fails, some of us feel strongly enough to actually vote for Hillary on the theory that it would be easier for an intact conservative movement to hold her in check than for a Republican party in Trump's thrall to control Trump's lifelong liberalism. Others would follow the course I intend to: simply staying home on election day.
Whatever happens, Trump would lose in November. And if by some diabolical miracle he won, the chances of his administration being anything other than an unmitigated disaster are nil. Preparations would immediately begin for a challenge to a thoroughly discredited Trump in the 2020 primaries- if, that is, Trump hasn't organized his own cult-of-personality political party by then.