The Titanic was a beautiful ship. It was a colossus—the culmination of decades of wisdom and design. It was financed and booked by the world’s rich and famous. It was unstoppable. And because it was, it steamed full ahead. Until it sank.

Democrats are this week beginning to freak out that Hillary Clinton is their Titanic, and to debate whether they might be better off on this 2016 political crossing in a less awesome, but more prudent, boat. The debate is overdue. The Clintons are masters at projecting invincibility and lulling...