You can't really compare this race to the New York City mayoral race. For one, Schumer is the leader of the Democrats in the senate and needs Democrats to unseat Republican senators in order for him to personally gain power. For two, the NY mayoral race was between Cuomo and Mamdani. it was not between a Democrat and a Republican.

Chuck Schumer only did so once his preferred candidate dropped out and Platner became unopposed in the primary. And all he said was “we’re going to retake the senate,” not any kind of personal endorsement

Except that Platner wasn’t the candidate promoted by DNC leadership. That would be Janet Mills. The Democrats failed to find a candidate who wasn’t almost 80 years old and Platner is going to sail to victory in the primary because he supports Medicare for all, stopping weapons sales to Israel and trans rights, positions the Democratic Party is too cowardly to endorse. In spite of his many character defects, he’s much more in touch with what voters want than Chuck Schumer.