Like many effeminate men, Graham needs to live vicariously as a "me too" guy for some alpha male in order to feel masculine and validated. For years, the alpha male he latched onto was McCain.
With McCain out of the picture, Lindsey seems to have latched on to Trump. Disturbing? Certainly. But on matters of policy, it works in our favor.
The other (worse) explanation is that this is all an act to make sure he doesn't get primaried out by a more "Trumpian" candidate in 2020, after which we'll be back to the old, open borders milquetoast liberal Lindsey.
You really nailed it.
Romney is not masculine.
He said once “I love trump and trump loves trump.”
That confidence may have won him over