Why polarization? Sharp rightward move of the GOP

"political scientists Keith Poole and Howard Rosenthal, who have long tracked historical trends in political polarization, said their studies of congressional votes found that Republicans are now more conservative than they have been in more than a century. Their data show a dramatic uptick in polarization, mostly caused by the sharp rightward move of the GOP."

-- Thomas E. Mann and Norman J. Ornstein in "Let’s just say it: The Republicans are the problem."