Socialism

Because he's a leftist who's apparently sincerely promising to actually work for the benefit of ordinary people and appears ready to actually do that if he wins.

Democrats like to pretend that they're all of those things too, but really all they ever try to be is somewhat less awful than Republicans, which is an easy enough task that they can generally succeed.

But if they end up being compared with someone like Mamdani, they're going to be revealed for the complacent, corrupt conservatives they actually are.