What is significant is that Omar’s defenders don’t consider her prejudices about Jews as particularly disqualifying, morally or politically, at least not when weighed against the things they like about her (and hate about her enemies). As for her views about Israel, she’s practically mainstream for her segment of the Democratic Party — a harbinger of what’s to come as the old guard of pro-Israel liberals like Majority Leader Steny Hoyer gives way to the anti-Israel wokesters typified by Alexandria Ocasio-Cortez.

What is all this reminiscent of?

Oh, right: the early days of Trump, when millions of Republican primary voters heard the candidate denounce Mexicans as drug dealers, criminals and rapists, and said to themselves, “We like that.” The central lesson of the moral collapse that followed for the G.O.P. isn’t that conservatives are a uniquely perfidious bunch. It’s that partisans of any stripe are always susceptible to demagoguery, particularly when the demagogue refuses to back down in the face of outrage. Shamelessness has a way of inspiring a following, and Omar is in the process of cornering the market on the left.