This is what I’m afraid of:
He’s unable to pass any of his legislative priorities in Congress because of GOP opposition, which will be fierce. I guess it’s possible he learns how to compromise and he gets some laws passed, but that would infuriate his diehard supporters to whom he promised no compromise.
So either because of anger over a diluted agenda, or disillusionment because nothing gets done, he loses his core of support which, to my eyes, is based on the idea that we elect him and he’ll ride into Washington on a white horse and make everything good again.
Having lost faith in the very idea that voting can change anything, his former supporters don’t vote in 2018, and the GOP lockhold on Congress is further entrenched. And they stay home in 2020 because they no longer believe that electing a “progressive” president can break up the banks, ban fracking and Monsanto, overturn Citizens United, make peace in the Middle East, send Wall Street into a giant sinkhole, and deport the Koch Brothers, McDonalds, and Walmart. The left in general loses support, loses steam.
And we end up with Cruz or someone similarly grotesque as president in 2020, all 3 branches of the federal government are controlled by theocrats and every small gain liberals have made in the last century or two are rolled back one by one, and on and on till the day I die.
That’s pretty much what I’m afraid of.
Of course all the above puts aside the question of who has a better chance of defeating a Republican. People have strong opinions on both sides. I happen to think Clinton has a much better chance, but it's really all guesswork at this point.