They've spent a half century playing on bigotries, jingoism, biases, right down to pet peeves to get the numbers they needed to be more han a fringe. They've scorned compromise, railed against it, and preferred to smash norms and the status quo to push their agenda.

They are geared to eliminating opposition, not respecting it, and their cult has followed suit. They smashed the Rockefeller Republicans within their party, as a stepping stone to full-on attacks of the Democrats. So when the cultists got frustrated with the leadership for not not driving he Hated Enemy under, they went for another con man who again told them what they wanted to hear.

Trump didn't do shit to the Republican party. They built Trump's machine for him. They Built That, to use a slogan that proves the point.

...Southern Strategy/dog whistling, race-based gerrymandering, race-based voter suppression, sexism, anti-intellectualism, jingoism and xenophobia.

Republicans (retiring members of Congress or TV pundits) who are speaking out against Trump don't want to admit that, because it would mean they were complicit. I imagine some are feeling a strong sense of guilt, but most are probably just in denial.