A common meme in the press is that Pennsylvania, like the rest of the Northeast, has shifted to the left in the last 20 years. However, matters are more complicated than this. Pennsylvania has exhibited a consistent, three-to-five point Democratic tilt over the last 50 years. That means that if, for instance, a Democrat wins the national vote by five points, we can expect him to win Pennsylvania by eight to ten points. We saw roughly this in 2008. Obama won the national vote by about seven points, and he won Pennsylvania by about ten points, for a three-point tilt. This is right in line with the historical average of the Keystone State.

This statewide consistency masks major changes within the state. There have been two big developments in Pennsylvania's political geography in the last 20 years that have counteracted each other - so that neither party has really gained a net benefit on the presidential level. However, these changes have cut decisively against Arlen Specter. I believe they are key to understanding why he left the GOP.

For the last twenty years or so, metropolitan Philadelphia in the southeast has been moving to the Democratic Party. However, this movement has so far been countered by movement toward the GOP in metro Pittsburgh in particular and the west in general. That, plus the population growth of the strongly Republican, exurban counties of Lancaster and York, means that the state as a whole still votes for President as it has for fifty years.

We can appreciate this in the following map, which shows the shift in presidential voting from 1976 to 2004.

I have not updated this map for 2008 - but I can say that metro Philadelphia continued its movement to the left while metro Pittsburgh moved to the right. McCain did better than Bush in five of the seven counties that make up the latter. He did no worse in Allegheny County, where the city of Pittsburgh is located. And he did only a point worse in heavily Republican Butler County, which has voted for the GOP in every election but 1964.

The story of Philadelphia's movement to the left has been well-documented, and I won't repeat it here. What's happened in the west has not gone as noticed - but its political consequence has been significant. It's worth a brief discussion.

This part of the country was staunchly New Deal Democratic for decades following the Great Depression. Ronald Reagan lost every county of metro Pittsburgh save one in 1984. However, in the last twenty years the steel industry has all but disappeared - with only the Edgar Thompson Works and the Steelers insignia as the last vestiges of what used to be. As the industrial jobs have gone overseas, greater Pittsburgh has moved to the right. This is a movement that has also been exhibited in the tri-state area. George W. Bush and John McCain did well in southern and western Ohio, as well as West Virginia. It's not coincidental that John McCain and Sarah Palin made their final stand here in Western PA.

How does this relate to Arlen Specter? He's from eastern Pennsylvania. That's where his political roots are - and that area has been atrophying Republicans. In the last five years, the GOP has lost about 60,000 registered Republicans statewide. In metropolitan Philadelphia alone, it has lost about 100,000. In other words, outside of metro Philly, the GOP has not shed voters. [Although with the growth in the size of the overall electorate, it has lost standing relative to the Democrats inside and outside metro Philadelphia.] Western Pennsylvania voters - while they are now more amenable to and constitute a larger portion of the state GOP - do not have local ties to Specter, tend to be culturally conservative and thus more likely to disagree with him on big issues like abortion, and are generally part of the GOP's rise in a part of the country that has little connection to the old party establishment in the Northeast.

In the 2004 GOP primary every county in metro Pittsburgh voted for Toomey over Specter - and Specter failed to crack 40% in several of them. In the general election that year, Specter ran behind Bush in six of the seven counties in metro Pittsburgh, even though he won the state by almost ten points and Bush lost it by two and a half. In 1992 - the last time Specter faced a tough general election challenge - his opponent, Lynn Yeakel, won six of the seven counties that border Ohio. Additionally, Toomey defeated Specter in York and Lancaster counties in the 2004 primary. Specter's narrow victory in the primary depended entirely on him sweeping Toomey in metropolitan Philadelphia, whose declining importance in the statewide Republican electorate has now made Specter exceedingly vulnerable.

I think the big story - which I do not expect to be emphasized because many Beltway pundits don't know much about Pennsylvania politics, especially west of the Appalachian Mountains - is that the political dynamic in the Keystone State has shifted, not so much against the GOP (at least on the presidential level), but against Arlen Specter, who has - during his twenty eight years in the Senate - failed to develop a durable political connection to Western Pennsylvania. When he entered the Senate, metropolitan Philadelphia, his home base, was also the GOP's base in the state. In 1980 four of the five counties in Philadelphia voted for Reagan while five of the seven counties in metro Pittsburgh voted for Carter. This has basically been inverted in the last quarter century - and while neither party's presidential candidate has been better off statewide for this shift, Arlen Specter has personally been on the losing end.

The interpretation from the wise political sages in Washington, D.C. is inevitably going to be about how the hardened, conservative rump Republican Party is so intolerant of a moderate like Arlen Specter that he had no choice but to bolt. However, this is quite an oversimplification. There is a big geographical component to this story: the west has become more important in party politics, and Specter has long been weak in the west.

One of the great all time books in political science is called Homestyle, by Richard Fenno. Professor Fenno tracked a dozen or so members of Congress in the 1970s to learn how they interacted with their constituents. He noted that incumbents frequently run into trouble when the demographics of their districts shift. If they don't shift with them, they can lose. The demographics of the Pennsylvania GOP have shifted on Arlen Specter - the base of the party has moved away from his home area where his personal ties are strongest. This left him extremely vulnerable heading into the 2010 primary. With Toomey positioned to take advantage - Specter switched sides.