The Southern Poverty Law Center’s co-founder, president and legal director have all departed amid a scandal over publicly unspecified workplace conduct, reportedly involving racial prejudice and sexual harassment. But the SPLC’s work itself is scandalous. The group falsely maligns ideological opponents in an effort to crush them rather than debate their ideas honestly. I know, because in 2016 the SPLC branded my organization, the religious-liberty nonprofit Alliance Defending Freedom, a “hate group.”

The label has forced itself...