Democrats aren't fighting him, the DNC (Democratic National Convention) is. The party doesn't want him as their representative, because he doesn't match their values. But he represents what the Democrat constituents actually want and the DNC fears losing control over their constituents, so they want to ensure he loses the race.
The DNC and the RNC basically want the same thing, except the Democratic Party is much more subtle about it. The US doesn't actually have a left-wing party. Both our major parties are right-wing by global standards. So any truly left-wing candidates scare them, and they'll fight tooth and nail to keep them out of elections.
Even Cuomo, who was completely stomped in the primaries, is still planning to run in the election as an independent. Their hope is that his run will split the party and make Mamdani lose. If the Democratic Party can't get their man in the election, then they'll throw the race and take everyone down with them.
The DNC and the RNC basically want the same thing, except the Democratic Party is much more subtle about it.
The old RNC. The new Republicans have taken over and actually do want all the fascist hateful stuff. The old guard courted crazies and those crazies took over.
He's popular with the voters. He's going to stomp New York, and then he'll start looking at the presidency. And he might win that, too. And then they'd have to pass bills, and their donors would be mad.
He said billionaires shouldn't exist. If billionaires didn't exist who would fund their campaigns running on platforms that allow billionaires to hoard wealth unimpeded?