HERE’S a political puzzle. A year ago, educational vouchers and tuition tax credits were almost a taboo subject. Today, the notion of public money for private and parochial schooling has made a startling comeback. Conservative advocates have suddenly been joined by liberals. In Wisconsin, Tommy Thompson and Polly Williams—the Republican governor and a black Democratic state legislator—have together fashioned legislation that is sending approximately four hundred Milwaukee inner-city students to attend private schools with public money. In Oregon, dissatisfied parents across the political spectrum have put the question of tuition tax credits on the November ballot. And in Washington John Chubb and Terry Moe, with the imprimatur of the Brookings Institution, have made national news with their barely disguised voucher plan. What explains the change?