I left the GOP in 1992. Before then, I had happily voted for Reagan and Bush 41. I'd been raised as a strong (and racist) conservative in the MS Delta. My family there was scandalized when I told them I was a strong Obama supporter - not so much because he was a Democrat, but because he was a Black man wanting to take up residence in the White House.
That being said, what you see today in the GOP is the result of nearly four decades of brainwashing, from right-wing talk radio dominated by people like Rush Limbaugh, then the rise of Fox News, and then the rise of right-wing media on the internet...all of which was accompanied by the rise of right-wing evangelicals.
Without exception, one of the core messages by every one of those sources was "Don't ever listen to liberals or the Democrats - they only lie, and you'll only get factual information from us." The evangelicals made it worse by essentially making it a sin to support anything liberals supported - like, say, action against global warming.
Worst of all was the fact that right-wing pundits and preachers became kingmakers - absolutely no Republican could win a primary without the aegis of the likes of Limbaugh or Falwell Sr.
As a result, you've got 40% of the American population - most of whom honestly, truly BELIEVE that they are good, honest, hardworking, well-meaning people (and who believe they aren't racist at all) - because they get all their info from right-wing sources...
...and so it is increasingly impossible for them to even conceive that maybe, just maybe the liberals might be right about anything at all.
It's as with any cult - some escape, but most are good people who don't recognize the wrong they do for what it is.
Alexander Solzhenitsyn once said, "In order for men to do great evil, they must first believe they are doing good." This was in his book "The Gulag Archipelago" - he understood that the guards and others who supported the gulag system really believed they were doing the right thing.
That's what we're watching in America today.