The racist, antisemitic chat messages between Young Republican leaders published last week in POLITICO were yet another example of the right’s efforts to “own the libs,” albeit one meant to remain private. This kind of norm-busting isn’t the only reason Americans are polarized — the left’s identity politics soured even many Democrats. But while President Donald Trump has raised “owning the libs” to an art form, he didn’t invent the practice of dogmatically disrupting suffocating progressive orthodoxy. Nor did the Tea Party or “birthers,” whose hostility toward the first Black president ignored standard rules of evidence. Nor did Pat Buchanan...