Historically the Republican candidates have demonstrated enviable party discipline and fallen in line after the nominee has been chosen, but Trump and the rest of the MAGA crowd have shown themselves to be so duplicitous that they won’t even honor agreements made with other Republicans. The more mainstream candidates like Pence may just figure that this time around there’s nothing in it for them, that they won’t get anything from Trump in return for their endorsement, not even goodwill.
Yeah, I’m old enough to have watched Enterprise when it was first aired, and I always went to grab a snack from the kitchen or something while the theme played.