Assuming Trump is the GOP nominee...will 2nd place in the primaries endorse him?

[Pennsylvania Deplorable]

Did Randall Terry and John Wolfe endorse Obama? I don't think so...

My *guess* is that at least one person will be challenging Trump in the primaries who will have enough of an organization to get ballot access in more than half the states.  I think even a McMullin could pull that off.  That's my guess, but I wouldn't call it a sure thing....

I think there will be several challengers. Kasich, Cruz, Martinez, McMullin and maybe even Romney.

I highly doubt that. The majority of republicans approve of Trump, so he'd have to mess up really badly for anyone important to run against him (as doing so would be political suicide for them otherwise). Even if there was a sizable anti-Trump wing, splitting it up would make the task of ousting an incumbent president in the primaries even more impossible. I could see McMullin running as a protest vote and maybe taking Utah but that's about it as of right now. Perhaps Glenn Beck would try to replicate Cruz's success in Iowa, but he's a nut and most evangelicals support Trump, so he'd only win the hardcore Roy Moore loving types.

Obviously, anyone who would run against Trump would come from the #NeverTrump faction of 2016 and not endorse him in 2020 either.