Data from the Brookings Institution and the American Enterprise Institute track the ideological positions of the Republican and Democratic caucuses over time. House Republicans have undergone a much more dramatic shift to the right during the past 35 years than House Democrats move to the left.
Interestingly, the U.S. Senate has not undergone a similar shift and has hovered near the center of the ideological spectrum for the last 65 years.
http://www.nationaljournal.com/congress/the-house-is-the-most-conservative-it-s-been-in-more-than-60-years-20130715
