rightwing forces of state repression can easily push around a left that's been trained and groomed to function as entirely beholden to corporate sponsors and big money donors.
That's not a "left" at all. There are traditionally three groups that vie for power, royalty, merchants, and the people. The left serves the people, Democrats serve the merchants, and Republicans are the royalists.
The Democrats may represent the left some day, but it's not who they are right now.
I predict Mamdani and Trump will have similar arcs through the media landscape. The media hated Trump but just couldn't stop paying attention to him, even though every time they attacked him it only made him stronger. Now they are all bending the knee to him.
The same exact thing is happening with Mamdani, but this time for all the best reasons. Mamdani's campaign might easily become a nationwide avalanche.