Republicans took a thorough beating in Tuesday’s elections, and no one should sugar coat the results because the voting was confined to a few states. Democrats came out in droves to send a message of opposition to Donald Trump, and GOP candidates were swamped in the undertow. While the cliché is not to read too much into an off-year election, this defeat was broad and deep enough to signal that Republicans will struggle to hold Congress next year.

The Democratic sweep in New Jersey and especially Virginia is in that sense...