Personally, I think this has been building since the day Limbaugh went national.
He didn't just feed them fully one-sided propaganda in a highly entertaining (he began as a deejay and brought a deejay's creative entertainment element to it) way. He gave them a thought process. So it wasn't just telling them what to think, it was showing them HOW to think. So they looked at everything through the prism of that thought process.
Then came the radio copycats, then FOX, then the internet to reinforce and amplify. Trump saw it. As another poster has said, Trump took one look at the talk radio audience and said, "THIS is gonna be EASY".
And here we are.