The Trump era has seen an erosion of the distinction between journalism and partisan politics, with much of the mainstream media in open opposition to the president. “Balance has been on vacation since Mr. Trump stepped onto his golden Trump Tower escalator . . . to announce his candidacy,” New York Times columnist Jim Rutenberg wrote in August 2016.

Three years later, the holiday continues. Slate last month published a leaked transcript of a staff “town hall” at the Times. “We built our newsroom to cover one story,” executive...