The author makes this too complicated. Republicans supported Nixon until polling showed it was clear that they would lose if they continued to tie their electoral fortunes to his presidency. Despite 2018 results, Republicans by and large don’t believe his continued Presidency and behavior will have a negative effect on them in the 2020 election. They’re wrong. 2020 will be more like 2018 than 2016. If and when Republicans recognize that fact, they’ll turn against Trump.

For most Republicans or Democrats — electoral survival continues to be the primary motivation.