Graham sounds like he's on the Trump Train now:
http://www.politico.com/story/2017/04/lindsey-graham-praises-trump-237361Of course, Graham will still be holding public office in 2020, whereas Bush, Kasich, and Pataki (presumably) will not. My guess is that in 2020, you'll still have a fair number of Republican former politicians (like the Bushes) who won't endorse Trump. They might not explicitly state that they'll vote Democrat or 3rd party, but they'll just refuse to comment on the race, following what George W. Bush did in 2016. But among Republican politicians still holding office, almost all will endorse Trump.
At least, that's what I see in the general election. The primary is less clear. If Justin Amash runs a longshot challenge against Trump in the GOP primaries, does Rand Paul definitely endorse him? And would he actually campaign with him, as a surrogate? Or would he officially remain neutral? I could imagine quite a few GOP politicos officially remaining neutral in the primary (even if the primary challenge is a super longshot and everyone reading the polls can see that Trump will be re-nominated), but then endorsing Trump in the GE once he's secured the nomination. That might be the default position for folks who still can't stand Trump, but don't feel like they can go full blown #NeverTrump this time.