Donald Trump is not his platform. His platform is widely embraced by his party. He is not. Prominent Republicans have been either distancing themselves from him, or leaving the party outright, as George F. Will did. And Trump is underperforming. Political science reference time.
"Out of step, out of office," by Canes-Wrone, Brady and Cogan (American Political Science Review, 2002). Basic question: how much does ideological "extremism" affect congressional incumbents' vote shares? Answer: a little, not much. They lose something, but they still tend to win reelection, basically because there is too much other stuff going on. There are a lot of other attempts to measure the effects, but for a variety of reasons, this one gets cited the most.
And in a presidential race? I've mentioned this before, but, um, DDRRDDRRDDRR, and so forth. How many elections would you have to flip to get that pattern from 1944 to 2012? One. 1980. (With an asterisk next to 2000...). Then, you've got the state of the economy, and other such stuff affecting presidential elections, and the effects of policy platforms tend to be pretty small in presidential campaigns too.
Then there's Trump. Dude's just weird. And that's a technical, political science term. So, part of what is going on here is that state, local and even congressional races are subject to normal forces, allowing Republicans to do just fine, even if some might consider their platforms "extreme" because "extreme" platforms aren't all that costly in electoral terms. Trump is losing. Why? It ain't the extremity. It's his Trumpiness.
How "extreme" is Trump, in ideological terms? He isn't an ideologue at all. His history of policy statements indicates no commitment to conservative causes. He adopted conservative language, rather, to run as a Republican. However, that means that his current stances are little different from Republican orthodoxy in most ways. Whatever electoral penalties he is facing, then, are not policy-based. His greatest polling weaknesses have tended to be on questions such as fitness for office, temperament, and such.
And that returns me to a theme from Part I. Predictions of DOOM for the Republican Party? Nope. This is just a party probably (76% chance, as of this morning's betting odds) throwing away one presidential election by nominating one bad candidate.
Ted Cruz, John Kasich, and a few others are enjoying their schadenfreude.