This. This is the real metric, since IIRC Karl Rove managed to swing Pennsylvania twice for Dubba in ‘99 & ‘03. Something that hadn’t happened since Nixon’s ‘72 landslide.
Obama’s wins were of course historic, but also were outliers, I believe, because of his race, which in and of itself changed the dynamics, as well as him being a somewhat unknown commodity. That being the case means that the true comparison should be with the pre-Obama elections.
Republicans have been gaining traction (and seats) ever since the other Clinton was in office during his first term. The Democrats have been declining for quite some time and the trend continues. This means that the Democrats are in free fall and they’re running scared.