Texas has become one of America’s fastest-growing states, thanks in part to its lack of a state income tax. So it was encouraging last week when Lone Star State voters made it even more difficult to impose such a tax.

The Texas constitution since 1993 has barred the Legislature from imposing an income tax without the approval of voters in a statewide referendum. But with progressives working hard to turn Texas into another California, voters decided to raise the bar. Proposition 4 changes the state constitution to require...