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When Sen. Arlen Specter of Pennsylvania switched to the Democratic Party this week, the response from Republican leaders was unequivocal: Don't let the door hit you on your way out.

Of course, they said that while standing, metaphorically speaking, in a building with no roof, broken windows, and collapsing walls.

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.

The backlash against a rigidly conservative president produced a shrunken Pennsylvania GOP so doctrinaire that the eclectic Specter had little chance of winning its support.

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. In the Senate, for instance, Democrats hold 22 of the 58 seats representing the 29 states that twice voted for George W. Bush. And just 40 percent of self-identified Democrats consider themselves liberals, according to Gallup polling; the rest identify as moderate or conservative.

By contrast, the GOP is becoming an increasingly monochromatic party, dominated by the most conservative voters and regions. This process enormously accelerated under Bush and Karl Rove, who built their governing strategy on energizing the Republican base rather than on expanding it by courting swing voters. Today, Democrats hold their largest advantage in party identification over Republicans since President Reagan's first term, and 70 percent of the shrunken GOP core identifies as conservative. After Specter's leap, Republicans hold just two of the 36 Senate seats in the 18 mostly affluent and secular "blue-wall" states that twice voted against Bush -- and that have now voted Democratic in each of the past five presidential elections.

Specter's defection shows how this contraction feeds on itself. The Pennsylvanian changed parties largely because he calculated that he could not survive a 2010 Republican primary challenge from conservative former Rep. Pat Toomey. This was a reasonable conclusion: Specter's base of moderates abandoned the GOP in huge numbers during Bush's second term. Their departure has left behind mostly conservatives receptive to Toomey's hard-core small-government message. In other words, the backlash against a rigidly conservative president produced a shrunken Pennsylvania GOP so doctrinaire that the eclectic Specter had little chance of winning its support. Now he's a good bet to win re-election next year as a Democrat.

Specter's switch shows that Republicans haven't yet paid the final bills for Bush and Rove's insular strategy. That price has been especially steep in the Northeast. In 1988, George H.W. Bush won eight of the 11 states from Maryland to Maine. Even as recently as 2000, Republicans won 40 percent of the House seats and held eight of the 22 Senate seats from those states. But amid the younger Bush's polarizing, Southern-inflected conservatism, Northeastern Republicans fell through the floorboards: They now hold only 18 percent of the region's House seats and, since Specter's switch, just three of its 22 Senate seats. In 2008, Barack Obama won all 11 Northeastern states and a combined 60 percent of their votes. Some weakened individual Democrats may provide isolated electoral opportunities for the GOP in 2009 and 2010, but across much of the Northeast, Republicans are now about as relevant as Whigs.

Shrewd former Rep. Tom Davis of Virginia, who chaired the National Republican Congressional Committee, calls Specter's defection a "devastating blow" that will send a "bad signal" of ideological intolerance to the moderate white-collar suburbanites the party must recapture if it is to threaten the Democrats' congressional and Electoral College majorities. "The dilemma for Republicans is, are we going to become a coalition, or are we going to be a private club?" Davis says.

Republican National Committee Chairman Michael Steele made his choice clear when he immediately issued a statement condemning Specter's "left-wing voting record." Rush Limbaugh and other conservatives quickly echoed Steele's good-riddance message. Any GOP coalition too narrow to welcome voters who share Specter's moderate views is almost certainly too narrow to dislodge the Democrats. But for many on the right, it seems, contraction and powerlessness are a reasonable price to pay for purity in the crumbling Republican clubhouse.