Sorted

"Historically, Specter's move is best understood as the signal event in the next stage of what the journalist Ronald Brownstein has called 'The Great Sorting Out'—a gradual but massive sorting of voters and elected officials that has brought their partisan affiliation into close alignment with their ideology. In The Second Civil War (2007), Brownstein showed how over the last four decades 'conservatives' have increasingly become Republicans and 'liberals' increasingly Democratic—turning these once-motley coalitions into relatively uniform ideological vehicles. In 1970, 35 percent of Democrats called themselves liberal and 26 percent called themselves conservative. By 2000, 52 percent were liberal, only 17 percent conservative. The GOP saw a mirror-image change. The mere 9 percent of Republicans who called themselves liberal in 1970 dwindled to 6 percent in 2000; the 63 percent conservative portion grew to 77 percent."

David Greenberg in Slate explains the context of Arlen Specter's defection.