President Donald Trump’s recent focus on urban crime presents a classic dilemma for Democrats. If they point out that his lurid portrait of a violent epidemic is inaccurate, they risk turning off voters who agree with him on an emotional level. But if they go along with it, they risk legitimizing his power grab. Trump and his Republican allies clearly hope to ride this issue all the way into next year’s midterms, so Democrats need to come up with a counteroffensive soon. Fortunately for them, there is a group of Democrats who know exactly how to run — and win...