Jacob Hacker and Paul Pierson, Off Center: Chapter 1, "Off Center"

There is greater political polarization almost entirely because the Republican Party has become more extreme at all levels: the base of party activists, the rank and file congressional membership, the presidency, and the top political bosses and consultants. There has not been a corresponding trend among Democratic Party activists and members of Congress. Democratic activists have not become particularly more liberal, and Democratic members of Congress only slightly so.

Nor is there a corresponding trend among the public at large. Polling shows that the public has grown neither more polarized nor more conservative. Furthermore, the polling data probably understates the gap between the increasingly radical Republican party and the electorate. This is because of the somewhat narrow range of issues polled and because of the ability of focused Republican political campaigns to shape opinion on a few issues.