In one sense, Specter's defection merely continues a generation-long trend. Since the 1960s, each party's electoral coalition has grown more ideologically homogenous as conservatives have migrated away from the Democratic Party, and liberals and moderates have moved away from the GOP. That ideological resorting has thinned the ranks of Republican House and Senate members from left-leaning areas such as the Northeast and the West Coast and has culled Democrats from conservative regions, principally the South.
This ideological and geographic sorting-out has narrowed each party's reach. But Democrats in recent years have maintained a broader coalition, both in Congress and among voters, by demonstrating more receptivity to diverse views.
... for many on the right, it seems, contraction and powerlessness are a reasonable price to pay for purity in the crumbling Republican clubhouse.
http://www.nationaljournal.com/njmagazine/nj_20090502_6655.php