It’s impossible to measure the weight all these liabilities would bear upon a Sanders candidacy. The quality of a candidate is not the only, or even the main, determinate of election outcomes, and having popular views is only one factor in the quality of a candidate. Still, political science has generally found that, all things being equal, the electorate tends to punish ideologically extreme candidates. You can peruse studies finding such a conclusion here, here, and here. Again, none of this says the more extreme candidate always loses, merely that extremism creates a handicap.

For obvious reasons, the Democratic Party’s left wing has always resisted this conclusion (as has the Republican Party’s right wing.) But Hillary Clinton’s surprising defeat created an opportunity for the party’s left to promote an alternative theory for how the party could and should compete. It deemed Donald Trump’s win a sign that capitalism had created such distress that voters were now rejecting conventional politicians altogether and open to radical alternatives who might promise to smash the failing system. Indeed, by this reasoning, Democrats would do better, not worse, by nominating more left-wing candidates, who could distance themselves more credibly from the discredited Establishment.

Yet this theory has had two clear tests, and failed both of them spectacularly...