Fact-checking Trump’s lies is essential. It’s also increasingly fruitless.

Daniel Dale met President Trump’s convention speech with a tirade of truth Thursday night — a tour de force of fact-checking that left CNN anchor Anderson Cooper looking slightly stunned.

The cable network’s resident fact-checker motored through at least 21 falsehoods and misstatements he had found in Trump’s 70-minute speech, breathlessly debunking them at such a pace that when he finished, Cooper paused for a moment and then deadpanned, “Oh, that’s it?”

So, so much was simply wrong. Claims about the border wall, about drug prices, about unemployment, about his...