Why did the voters put Donald Trump, of all people, in the Oval Office? ...If he does not see what Trump has to do with the Republican Party, then he will never understand why the voters elected him.
Trump hijacked a party to which he did not, in any meaningful sense, belong. (And still doesn’t.)
Trump ran as a Republican, and was nominated by Republicans. That is enuf to make him a Republican. Beyond that, he has taken Republican policy positions on judges, immigration, taxes, regulation, health care, and many other issues. These positions are well-known, and are precisely why the Democrats hate him so much.
When Trump started campaigning two years ago, a lot of pundits were not sure what to make of him. What is there excuse now?
At this point, Trump is as Republican as anyone. Those not supporting him might not be Republicans.