"In his 1950 book The Liberal Imagination, Lionel Trilling said that “in the United States at this time liberalism is not only the dominant but even the sole intellectual tradition.” Liberalism was no less the dominant political tradition; a coherent conservative opposition had yet to emerge. Over the next 60 years, however, the liberal imagination lost its hold on the American mind. In October 2009 Gallup found that just 20 percent of Americans described themselves as liberals; twice as many called themselves conservatives.


What happened?"

Michael Knox Beran has the answer. It's a long answer but sounds about right.