In their first major act together, Ms. Hochul and Mr. Mamdani last week unveiled the first part of a much-anticipated plan to provide free child care in the country’s largest city and one of its most populous states. The initiative, which will greatly expand the state’s safety net, could take years and billions of dollars to carry out fully. But the pair are together giving Americans a clear vision for what Democratic governing can look like, evoking parts of the New Deal era in a bracing contrast with the growing cruelty and wantonness of the Trump administration.