They think everyone are idiots.
I think the tipping point was '92. Back then, a presidential candidate's history of illegal drug usage was a big deal, and should have been a deal breaker.
When Clinton addressed the nation to say, "I smoked, but didn't inhale," I was personally insulted that a candidate would tell such a ridiculous lie. I thought, "He must think the electorate is a bunch of idiots." That November, he was proved to be right.
According to 81 million "ballots", and the natural distribution on the Bell Curve, at least half are, demonstrably.