When House Republicans unveiled their plan for tax reform this month, Democrats offered a curious response. Rather than give their own proposal, they held a mock hearing—a show trial, really—on Kansas’ 2012 tax cuts. Democrats see my state as proof that supply-side economics doesn’t work and that the GOP’s national tax reform will lead to budget cuts and enormous deficits. But the comparison is less a cautionary tale than a tall tale. I was Kansas’ state treasurer from 2011-17, and no one watched the dollars flowing in and out more closely than I did. Consider a few facts: First, the...