The decision by Jay Jacobs, the party chairman, puts him at odds with Democratic primary voters and New York’s governor.

© Dave Sanders for The New York Times

Zohran Mamdani, the Democratic mayoral nominee in New York City, received messages calling him a terrorist and telling him he was unwelcome in the U.S.

© Pool Photos by Dean Moses

I am confident that Mamdani has the courage, urgency and optimism New York City needs to lead it through the challenges of this moment.

Even if Mr. Mamdani is as awful as his opponents insist, campaigning against an opponent’s flaws works only if you don’t seem even worse.