Remember all that angst and anger expressed by progressives that President Trump would ignore judicial orders, rig election laws, and maybe even refuse to give up power if he loses in 2020? We’re still waiting for any of that to happen. But that hasn’t stopped Democrats from stretching the Constitution to defeat Mr. Trump.

The latest example came Tuesday from California when Governor Gavin Newsom signed a law that would bar Mr. Trump from the state primary ballot unless he discloses his tax returns. That’s right. California...