I can only see Trump dropping out for 2 reasons:
1) He has a major physical or brain health problem (massive heart attack, stroke, open onset of Alzheimer's Disease which his father had) where he resigns or others have to invoke the 25th Amendment. Lyndon Johnson in 1968 quit his re-election campaign in part from his health.
2) If overwhelming physical or digital documentation of massive corruption of Trump closely before or during his term of office is produced by the ongoing investigations by House Committees and others that lead to Impeachment proceedings.
To that I'd add he has to make a choice between running and releasing tax returns. He will fight to the bitter end any state that makes full release a condition of being on the ballot. If the Supreme Court sides against him, he will successfully pressure the GOP to give the Democrats anything they want in exchange for passing a federal law overruling state tax return release requirements, but the Democrats won't bite, and Trump will fold.
I'm not sure we will ever get to #2, partly because short of having high-quality recordings of him explicitly ordering an assassination, there is nothing in Trump's past or current behavior that the GOP isn't willing to forgive in the name of retaining power. Anything else a House Committee finds, the Senate will find a way to cast doubt on to give themselves an excuse not to publicly renounce him... He could even dump Melania, date AOC, and get away with it.
Even if he sees a loss coming, it's better for him to lose the election, than to not run. He/his supporters can always claim there was a conspiracy against him, that way.
With his claims that his first two years of presidency were robbed by the Special Counsel's investigation, I fear how Trump would react to a loss. For the first time ever, I can see a situation where the incumbent president refuses to accept the results of the election and vacate the White House... For the sake of avoiding a Constitutional crisis, I would very much prefer a Trump who quits, whatever the reason may be, over a Trump who loses. The former's ego will not be as bruised as the latter's, and he could still claim that he could have won if he had run...
This will make way for Nikki Haley to become Trump's running mate and will win back the women vote while keeping the evangelical vote in tact. I think this is why Nikki left, it also distances her from any future controversy in the first term.
She is far too smart to accept a VP nomination. Consider her options (in no particular order):
- Trump wins with Hailey as VP - she runs for president the next elections, likely unable to re-create the Trump fan base around her, and too bruised from her proximity to Trump to win back the suburban never-Trump vote she'll need.
- Trump wins with Pence as VP - she primaries Pence and wins, close enough to him ideologically to be an acceptable substitute, while retaining a shot at winning back the suburban votes.
- Trump loses - she is free to run in the next primaries.
Her odds of becoming president are much higher in the last two scenarios.
The Trump/Johnson special relationship: Special people on both sides of the Atlantic