I don't see it that way
People like Nader and Sanders put their own political aspirations ahead of what was good for the country and helped cause Dem candidates to lose elections, even though neither of them had a chance to win had they gotten nominated.
I think that the problem the Dems are currently faced with, is that the progressive wing wants to drag them too far to the left and advocate causes that too many centrists and independents want nothing to do with.
Personally I "thank" (blame) the extreme left for trump being in office.
Partially anyway.
The ignorant scum on the far right obviously shoulders most of the blame.