Chuck Schumer and Kirsten Gillibrand, who also played a big role in recruiting Mills and Stevens, refused to endorse the Democratic nominee for mayor of New York last year. They set the "Dem pols can refuse to endorse Democratic nominees who are insufficiently centrist/pro-Israel" precedent.
Janet Mills and Haley Stevens were two of the big candidates that Chuck Schumer recruited to run in this cycle. Mills refused to endorse Troy Jackson until a couple of days ago when she tepidly said she voted for him, and now Haley Stevens is endorsing the Republican nominee.