Democratic presidential candidates Sens. Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.) plan to "finance longer-term entitlement growth" by allowing the tax cuts implemented by President Bush to expire, but these proposals "fail budget arithmetic and economics," John Cogan, a senior fellow at the Hoover Institution and former deputy director of the Office of Management and Budget, and R. Glenn Hubbard, dean of Columbia Business School and former chair of the Council of Economic Advisers, write in a Wall Street Journal opinion piece...