I think that the Republicans have been doing well in the mid-term federal elections because most Democrats have no clue what a Senator or Representative actually does. In their authoritarian mindset they tend to think of the president as a king and senators and reps as his underlings. They don't know that the president cannot hire or fire them at will.
A notable exception is in 2006 when the Dems rode the truther wave into congress. The idiot demograph of the US went to the polls in droves and voted Democrat simply because they were convinced that a Republican president planted explosives in the WTC and other such idiotic nonsense.