A portion doesn't realize, and another portion is likely actively in favor of this outcome, as oligarchies are pretty good for the oligarchs (at least for a while). Unfortunately the first portion has the votes, and the second has the money; that's just the way things are at the moment.
The issues the GOP are getting those low income votes on are total fabrications (yeah, sure, coal will come back, even though no one's investing in it; it was the environmentalists that cost those mining jobs, not the robots that replaced the miners; "We're Pro Life", unless you've already exited the womb), but it works, apparently.
I'm really not sure what steps we can take to increase the level of rational, critical thought in this discussion. People read a headline, get the endorphin rush of reading something that confirms their own biases, retweet and then stop thinking! I've noticed this bias in my own behavior and have taken steps to correct it - if an article just feels good to read, I must double check that its sources are well cited and its points are correct, otherwise it gets a mark of questionable credibility in my brain.
I don't really know how to convince others to do the same...
But then the GOP do have constituents, via a continued enhancement of the Southern strategy, to get people to vote against their economic interests. Of course there simply aren't enough plutocrats to win elections, so they have to rile up some masses with various promises, rhetoric, and occasionally throwing them red meat.
You might re-examine that belief in light of the last election...
We are being converted into an oligarchy. The administration is backed by a demographic that either doesn't realize this, or doesn't care. They unabashedly silence their opposition on the floors of Congress and the Senate all the time. Can't get your legislation through? Change the rules.
The GOP has no constituents. Nobody with a net worth under $10,000,000 matters to the GOP.