Sen. Arlen Specter's decision to switch parties in this 2010 bid for re-election will draw most attention for its immediate impact on President Barack Obama's legislative agenda in Congress. Once Al Franken takes his Senate seat from Minnesota, as seems increasingly likely, Republicans no longer have enough votes on their own to mount a Senate filibuster.

But there is a deeper story of long-term significance: Sen. Specter's decision is another nail in the office of the more moderate Republican philosophy associated with former New York governor Nelson Rockefeller. In the last dozen years, Democrats have won sweeping victories in the Northeast, with the region's Republican Party now on life-support.

After the1992 elections, Republicans in New England and New York collectively held 20 of 54 U.S. House seats and held at least one House or Senate seat in every state. The intervening 16 years have been devastating for Republicans in the region, especially in 2006 and 2008. New England's last Republican House member, Chris Shays of Connecticut, was defeated in 2008, and Republicans now hold only three of 29 U.S. House seats in New York and no U.S. Senate or House seats in Rhode Island, Connecticut, Massachusetts and Vermont.

The shift has affected down ballot races as well, in both New England and broader swathes of the Northeast. In 2006 Democrats took control of both chambers of the New Hampshire legislature, and in 2008 took control of the New York senate for the first time in four decades, gained monopoly control in Delaware and greatly expanded their margins in state legislatures throughout the region.

Pennsylvania has showed signs of moving this direction too -- it's been more than two decades since a Republican carried the state in the presidential race, and Democrats in the past few years have won the governor's mansion and a U.S. Senate seat. But John McCain invested heavily in Pennsylvania, and the state senate remains firmly controlled by Republicans.

Sen. Specter's move could be a signal to a number of moderate Republicans in his state to abandon their party. It's too early to tell, but quite possibly Pennsylvania's days as a potential swing state could be over for at least a generation, and Democratic reach into the state could rise at all levels. Republicans throughout the Northeast have to be wondering whether their party's tent is big enough for them -- and for majorities in their state.