There are plenty of things I disagree with Mamdani on, as I'm generally dismissive and critical of certain socialist ideals. But the idea that he poses any thread to democrats is laughable. They called Obama a socialist and communist. Same with Hillary, Biden, Kamala. It doesn't matter what the platform is, they will call you a socialist/communist.
If Mamdani is a terrible mayor and his ideas fail then we can revisit this, because that could have consequences. But a lot of his policies are pretty NY-specific technocratic shyt that will likely have immediate positive impact on the city. Allowing street vendors, bodegas, etc to survive without being fukked up hundreds of fines/fees/red tape? Vast majority of NY is gonna be happy about that lol.