In 1976 and '78, the Supreme Court ruled that when corporations and billionaires own their very own politicians, it is constitutionally protected "free speech" rather than bribery, which is how we used to define it. These Republican politicians are now wholly owned agents of the corporate and billionaire class, and they don't want the money to stop, so they have to keep supporting the guy who is pushing the corporate and billionaire friendly policies of deregulation and tax cuts.
It's really just that simple. As long as the Supreme Court continues to assert that there is absolutely nothing wrong with billionaires and corporations owning their very own politicians, odds are The GOP will simply continue to be an extension of the lobbying industry and the morbidly rich. With or without a billionaire in the White House like Donald Trump.