There is a minority of people who are actually completely out of touch with reality, so-called demagogues and populists, and to a less extent all politians: they are completely deluded in thinking that by repeating a point a thousand times they can mislead those perfectly informed masses.

As the saying goes: "Fool me once, shame on you. Fool me twice, shame on me." A large part of the American populace has been getting fooled repeatedly for decades.

It has not been established that they are voting against their own self-interests. You are just assuming you know what their interests are better than they do.