If Republicans succeed in limiting the state-and-local tax deduction, one hope is that this could finally inspire a come-to-Jesus moment in prodigal high-tax states. Democrats in Illinois ought to be especially chastened by new IRS data showing an acceleration of out-migration.

The Prairie State lost a record $4.75 billion in adjusted gross income to other states in the 2015 tax year, according to recently IRS data released. That’s up from $3.4 billion in the prior year. Many of the migrants were retirees who often flock to...