I've always found that to be a curious expression, which is always used by the left when people they think should be in their constituancy don't wind up there.
Typical statist paternalism. They know what is in me econonic interests, but I don't. It seems to me that having my bank account ransacked by the burdensome taxation of local, state, and federal governments, and then to be pissed away by them, is not in my economic interests, but what do I know? That idea probably comes from the same wrongheaded thinking that leads me to believe that I have gotten and kept a job based on my skills and hard work, when the truth is that it was probably someone in the government who was responsible. What an ingrate I am!
Don’t you know, “voting against their own interests” are fascist leftist code words for “won’t do as their told?”