As far as congress is concerned? gerrymandering!
I don't agree with that.
It's complex. Many reasons. Gerrymandering may be one. Other reasons include:
Jesus loves republicans more than democrats?
Belief that the republicans will lower their taxes?
Belief that democrats are weak on national security?
Belief that republicans are more fiscally responsible?
All of which are lies.
Being racists could be another reason as well.
It's not just one thing. It never is.