Actually, all you need to know is in that article.
The consistent refrain is “We will win because we are right.”
OK, for one thing, you aren’t right. Yes, history will vindicate you on some issues, condemn you on others, and some positions will remain forever controversial.
But that is contingent. More important, you don’t own Being Right. It isn’t in your DNA. Why should the successes (what they were of them) of left-leaning, charismatic, straight-edged Obama naturally translate into a victory for centrist, off-putting, corrupt Clinton? Just because they both put “(D)” after their names?
Thinking that you are right (and therefore designed to win) blinds you to your side’s weakness, but acting like you think you are right is a huge weakness; Trump and the Republicans mocked the Clintons. Hillary and the Democrats mocked Trump supporters, and that is plain suicide.
There is a great scene in the old movie The Sting. Robert Redford is glaring across a train station at the villain, Robert Shaw and mutters, “He’s not a smart as he thinks he is.”
Paul Newman leans into frame to say, “Neither are we.”