President Trump’s boorish manner tends to obscure a hard-to-swallow reality: He often has a point. The failures of LBJ’s War on Poverty are increasingly visible to anyone who cares to look. To paraphrase the philosopher Ernest Gellner, some failed practices can’t be reconsidered, because they already shape the way we think.

Mr. Trump follows the example of New York Mayor Rudy Giuliani, who provoked an avalanche of media hostility in 1995 when he mocked Rep. Charles Rangel for seeming indifference to the decline of his Harlem district. Like Mr. Trump, Mr. Giuliani was disparaged as a racist. But he wasn’t wrong, and his efforts helped inspire the revival of Harlem’s main shopping stretch on 125th Street.

Mr. Cummings has been in Congress since 1983, to little local benefit. Mr. Trump was condemned for a tweet describing Mr. Cummings’s district as “a disgusting, rat and rodent infested mess.” Critics focused especially on the word “infested.” But in a 1999 congressional hearing, Mr. Cummings said: “This morning, I left my community of Baltimore, a drug-infested area.”

Washington’s efforts to revive Baltimore have enriched local politicians but left the city in far worse shape than Mr. Rangel’s Harlem. The American Recovery and Reinvestment Act of 2009—the Obama stimulus—poured $1.8 billion into Baltimore with no discernible effect. The 2015 antipolice riots made matters worse.