I think the problem goes even deeper still, Keith: It’s not that so many Americans don’t know he’s lying; they do. The problem is that they are perfectly fine with lies.
So even when the media ‘does its job’ and busts him in lie, it makes no difference whatsoever.
Why do people no longer care about lies? Because they have learned that ALL politicians lie, have have always lied, and will continue to lie to them.
In these cynical (realistic?) times, we know that we are unrealistic to expect truth from any of them; so we simply look for the ones who tell the lies that we like.
They support Trump because he tells tells the lies they prefer.