Here is the #1 to understand about Donald John Trump - he really believes that whatever is coming out of his mouth in THIS moment is true. To him, "I have to be honest" is an honest reflection of an inherently honest soul. The gullible & wanting-so-hard-to-believe buy it because it's delivered pure, sans a semblance of guile.
And because he is never ever wrong, whatever Donald Trump says in the next moment is true, too, even it contradicts what he said sixty seconds ago.
And for over a year, that has been okay with party leadership.
It's widely reported that the final straw for Reince Preibus in his all-in hand holding of the GOP candidate was when DJT balked at endorsing Paul Ryan, a close friend of the RNC Chair. Then & only then did GOP leadership rear up & say, "No more!" When it hit one of their own.
Well, we're at mid August. Trump has gone beyond calling on Russia to look around to see if they have HRC's 33,000 missing e-mails. He's amped up to making a thinly veiled reference to taking out a President Hillary Clinton if she makes SCOTUS picks that rile his base. He has named President Obama - no less than three times in one rambly sentence - as the founder of ISIS.
And, I firmly believe, no one believes those statements & charges more than he does, at the moment he says them.
Folks, it's called DELUSIONAL.
addendum 11:33 a.m.: a friend nails it when she comments, "Another possible way to look at it is that the concept of truth has no meaning to him. He doesn't believe his statements are true, he doesn't believe they're false, the whole question is irrelevant. The question is what he can say that will sell, in that moment."