The agonizing fact for Democrats this summer is that President Trump appears to be gaining ground on domestic and foreign policy, while his potential challengers are quarreling and mostly spinning their wheels. Trump is taunting allies and defying Congress — and seemingly getting away with it. He isn’t just rewriting the political rulebook, he’s tossing it aside. And the painful fact is that the Democrats haven’t figured out a way to stop his forward momentum, even when they believe it’s taking the country over a cliff. Trump remains a divisive and unpopular leader who is vulnerable in 2020. But a...