General Discussion

I don't have any insider knowledge about this, but from my observation, Lander was a major player in Mamdani's mayoral campaign, and helped Mamdani to get elected. For going out on a limb and supporting Mamdani, the political payoff for Lander was that when he wanted to run for Congress, Mamdani returned the favor and backed him. This was an easy choice for Mamdani, because not only was he returning a favor, Lander was running to the left of Goldman, and if he won, would be an ally to Mamdani in Congress as they had both helped each other to win important races.

That's a pretty normal state of affairs in politics everywhere. I'm not sure this was a case of "Mamdani running a candidate against Goldman" was it? As far as I know it was Lander's choice to run. Democrats like Elizabeth Warren also endorsed Lander.