The real political defect of the Cain plan is that it imposes a new national sales tax while maintaining the income tax. Mr. Cain's rates are seductively low, but the current income tax was introduced in 1913 with a top rate of 7% amid promises that it would never exceed 10%. By 1918 the top rate was 77%. Part of Mr. Cain's appeal is his willingness to challenge political convention, and he certainly has with his tax proposal. Voters like that he isn't a lifetime politician but a successful business owner who has met a payroll and created jobs. But...