It's easy to reach for the "people are stupid" argument.
It's harder to ask, "Why are people still voting for this despite it seemingly being against their interests"
But once you do start asking that question, you'll find "they're just stupid" isn't really the only answer. At least 1 other answer would be they're responding to politics of other parties failing them too.
Is it the most rational decision for those people? No, probably not, but ignoring their motivations and chalking it up to stupidity or whatnot is really not going to solve anything - and, in fact, is only going to push those people further into what they believe. You should consider whether that's what you want.