He's not a great politician... but he is the one most likely to have done what he would've he said and said what he would have done... and those things are what would've made independents break for him if he was the candidate. And despite the cries of misogynists or Bernie-Bros from establishment-Justice-apologists he would've also gotten the progressive base energized... something that Clinton and her business as usual failed to do based on the amount of votes democrats got for PotUS this year compared to 2012 and 2008.
Maybe he won't be the right candidate for the tone of the election in 2020 but he was for 2016 and the DNC and their fvcking apologists fvcked it up for the rest of us for 4+ years (maybe decades depending on the SCOTUS)
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People in the middle who evidently responded to a white nationalist campaign aren't going to vote for a leftist. Where Obama succeeded was with the Clintons' centrist approach, and Bill's political charisma.
Sander's key demo was the youths, and it's not easy to get them to turn out.