Political Scientist Hans Noel has this commentary in the Washington Post, about U.S. political parties, ideology, and polarization. The thesis is that the U.S. has long been divided by ideology. If parties didn’t seem polarized in the past, that is because there were liberals and conservatives in both major parties. But in recent years, “sorting” has been carried out, so that today, virtually all liberal politicians are Democrats and virtually all conservative politicians are Republicans.
Noel has written a book, “Political Ideologies and Political Parties in America”, which will be in print in February 2014.