The early days of 2019—which is to say, the early days of the 2020 campaign—have been dominated by the Democratic Party’s left wing. Early big-name entrants Kamala Harris, Elizabeth Warren, Cory Booker, and Kirsten Gillibrand were all quick to plant their flags squarely on progressive soil.

On Monday night, President Donald Trump held a rally in El Paso, Texas. He chose the location based on his claim, delivered in last week’s State of the Union address, that a border wall had rescued the city from rampant crime. By the time Trump arrived, fact-checkers had demolished this lie,...