You should know this better than the US, but our "democrats" are center right for the rest of the world. The goal is to sound progressive but then act in neoliberal ways to appeal to donors, after the attention isn't on them. I call these "Establshment Democrats", more concerned with keeping the status quo and being a PR machine to the people than actually making policy that benefit the people.
That's why Mamdami can cut through by saying the things that Establshment Dems hated. And early on in his campaign when he gained momentum you can see the resistance against him by the Establishment, up to Cuomo decided to run independent after the primaries. I can't speak for the common person, but those actions speak a lot louder than any words Mamdami said.
There is a rift in the US Left, but I think it's one Estblashiment Dems had coming for a while now. If absolutely nothing else, the rampant destruction of the country by the Trump admin has absolutely activated people in ways not seen since 9/11. And when people are active, words aren't enough anymore. They want action, to not see military roaming their streets and kidnapping US citizens. They want to see actual ways to fix the economy as these trade wars sap at their wealth.
The collorary here is that the MAGA movement is also causing a rift in the US Right. There's definitely Esablishment Republicans that do not like this situation either. And there's the fact that all this is propped on one obsese, Dementia-ridden, 79 year old man. If/when he passes, there's going to be a huge power vacuum, and none of the headrunners are ready to fill that.
If anything, the split on the Right will be worse than the split of the Left, when it eventually happens. At least the Left is having new blood to try and push that rift from the bottom up compared to the house of cards that is Trump and everyone who tried bundling under him.