As the 2020 presidential contest begins, it’s worth reflecting on one of the issues that made Donald Trump president—religious liberty. And it’s worth remembering a moment that helped him to win.

The date was April 28, 2015, and the case was Obergefell v. Hodges—the case that created the constitutional right to same-sex marriage. But it wasn’t the recognition of gay marriage that cost Democrats. Public opinion had been trending strongly in that direction for some time. No, it was a moment buried deep in the oral argument…