To sympathize with these revolutions in the 1920s was acceptable, given their original high ideals. To do so after the Hitler-Stalin pact, or in the 1950s, is appalling. To do so in the 1980s is morally unfathomable.

I say all this not to cancel Sanders for past misjudgments. I say all this because the intellectual suppositions that led him to embrace these views still guide his thinking today. I’ve just watched populism destroy traditional conservatism in the G.O.P. I’m here to tell you that Bernie Sanders is not a liberal Democrat. He’s what replaces liberal Democrats…

There is a specter haunting the world — corrosive populisms of right and left. These populisms grow out of real problems but are the wrong answers to them. For the past century, liberal Democrats from F.D.R. to Barack Obama knew how to beat back threats from the populist left. They knew how to defend the legitimacy of our system, even while reforming it.