Ms. Harris’s tepid endorsement came during an interview with Rachel Maddow on MSNBC to promote her book, “107 Days,” about her failed presidential campaign. “Look, as far as I’m concerned, he’s the Democratic nominee and he should be supported,” Ms. Harris said. Mr. Schumer refused to endorse Mr. Mamdani on CNN over the weekend and would not give host Dana Bash a reason. He refused to back Ms. Harris in the 2024 presidential race because of the Biden administration’s support for Israel. “As proud Democrats and elected officials and New Yorkers,” Mr. Mamdani said at the time, “we endorse the Leave It Blank campaign.”