The critical sixth key was the contest key: Bernie Sanders’s contest against Clinton. It was an open seat so you lost the incumbency key. The Democrats had done poorly in 2014 so you lost that key. There was no big domestic accomplishment following the Affordable Care Act in the previous term, and no big foreign policy splashy success following the killing of Bin Laden in the first term, so there were
Bernie sanders was the missing key for the Hillary loss then?