Bad: Sanders beats Trump. It’s a close call, but I think the slightly better option might be that the Democrats nominate Bernie, who beats Trump. It could happen. Bernie’s weaknesses—his anger, his off-putting personality, his kooky pronouncements, his whiff of authoritarianism—are not all that different from Trump’s. And as for his contempt for capitalism and the free market, he would only be echoing the nationalist conservatives.
So what would happen if he wins? The radical left would not only win massive credibility in the Democratic party—and moderate Democrats would be expected to fall in line—but they would also be able to wield actual power.
The saving grace would be that it would probably not be enough power to get all that much of what they want. Probably.
Like Barack Obama after 2010, Bernie would probably be stuck doing all the stuff he thinks he can do by executive order, without being able to pass lasting legislation. And yes, I know I just used the word “probably” a whole bunch of times.