So how did it get that way?
Because it can't be the economic upturn during Reagan's time, because Clinton and Obama both had good economies as well, and even more so for Obama because he took a financial disaster and cleaned it up, putting us back on track.
I think mostly this isn't reflecting any sort of evaluation of historical performance but is just people's sense of which party cares more about their side of some issue.
Republicans hate terrorists louder and Democrats want to take your guns. The Republicans are the party of responsible fiscal policy and getting government out of the way so that the economy does better. I'm not sure anyone knows much about where the parties are on trade but Trump has certainly been vocally anti- trade deals. Republicans want to cut your taxes and Democrats want to raise taxes on the rich. Democrats are the only ones who even believe in government spending. Trump wants to deport millions of decent people and is a racist. Trump is a crazy person (foreign policy). Democrats are the only ones who believe in education, health care, and the environment. Republicans are much more pro-life than the average American.