Ronald Reagan believed the Republican platform rested on a three-legged stool. The three legs were fiscal conservatives, social conservatives, and national security hawks. Under Donald Trump, all three legs have been sawed out from under the stool. Less than a third of Republicans support all three legs now.
Fiscal conservatives from Reagan through Paul Ryan and Mitt Romney wanted to slash government spending in order to reduce the federal debt. Trump doesn't even fully understand what the federal debt is and is against cutting defense spending, Social Security and Medicare, which together make up the biggest part of the federal budget. He has never even mentioned debt reduction and signed an unfunded tax cut bill that made the debt explode. So much for leg number one.
No one would mistake Trump for a religious zealot. He never goes to church and uses the Bible only as a prop for photo ops. Before he got into politics, he was a Democrat and pro-choice. To the extent that he supports the religious right now, it is simply an act to get votes. He doesn't care about their issues at all. Appointing conservative judges is easy, doesn't cost him any political capital, and avoids making any enemies within the Party. He does care about immigration and crime, but not abortion or same-sex marriage. There goes leg number two.
How about national security? Reagan called the Soviet Union the "Evil Empire." Trump calls Vladimir Putin his good friend. He has no problem with Putin invading other countries and trying to annex them. If Leonid Brezhnev, Yuri Andropov, or Konstantin Chernenko had tried that, Reagan would have started World War III to stop them. He would certainly not have excused such behavior as Trump has. Oops, there goes leg number three. The platform is now floating in space, all by itself.
In effect, Trump managed to pull off a hostile takeover of the Republican Party by telling its voters that they are victims of the deep state and he would exact vengeance for them. Reagan was sunny and upbeat, talking about "Morning in America." It would never occur to him in a million years to call his supporters victims. Reagan greatly supported free trade, something Trump opposes. Just about everything the Gipper supported is anathema to Trump. Trump basically remade the Republican Party in his own image in his short 8 years as a figure in national politics, a pretty stupendous feat, when you think about it. Anyone who says Trump is not a brilliant politician hasn't been paying attention. (V)