Surely nobody can be tricked onto supporting positions contrary to his interests. That would be impossible, since people are rational, educated and perfectly informed.
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.
If someone continuously keeps getting tricked into voting against their own self-interests, with the belief that their vote will actually benefit them, then yes, they absolutely deserve contempt.
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.
If someone continuously keeps getting tricked into voting against their own self-interests
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.
Or then again, maybe they just have a different opinion than you on what is important to them?