The Left’s near monopoly over the institutions of opinion and information—which long allowed liberal opinion makers to sweep aside ideas and beliefs they disagreed with, as if they were beneath argument—is skidding to a startlingly swift halt. The transformation has gone far beyond the rise of conservative talk radio, that, ever since Rush Limbaugh’s debut 15 years ago, has chipped away at the power of the New York Times, the networks, and the rest of the elite media to set the terms of the nation’s political and cultural debate. Almost overnight, three huge changes in communications have injected conservative ideas...