One major political trend of the past generation is the rising number of Americans who identify as liberal. This has led to predictions of an “emerging Democratic majority” fueled by young and minority voters as well as a bunker mentality on parts of the right.

Yet Gallup reports that its comprehensive 2019 survey found the share of liberals declined, and the share of conservatives increased, for the first time since the 2011 burst of Tea Party energy. This took place despite Donald Trump’s relative unpopularity and polarizing...