Except for the fact that in the primary, *only* registered republicans have the chance to vote for or against him. In the general election, he also has to contend with Dems (which will obviously vote for Hillary) and middle-of-the-roaders (which will break about 50-50). The Republican base will not support him in adequate numbers, because he just doesn’t represent them or their values.
If Rubdy is the nominee, then Republicans did vote for him. So, don't say the base will abandon him. It was the base, who showed up during the primaries, and cast their ballot for him.