Connecticut’s progressive tax experiment has hit a wall. Tens of thousands of residents are fleeing for lower tax climes, which has prompted Democrats to propose—get this—paying new college grads a thousand bucks to stick around. Maybe they’re afraid an exodus of young people will turn the state Republican.

Hard to believe, but a mere 25 years ago—a lifetime for millennials—Connecticut was a low-tax haven for Northeasterners. The state enacted an income tax in 1991 that was initially a flat 4.5% but was later made steeply...