Pennsylvania is splitting in two.

It’s a phenomenon I’ve written about quite a bit as western PA has trended Republican and eastern PA trended Democratic over the past few decades.

I call this the Appalachia/Acela divide.

One of the more intriguing facets of this shift is the fact that most of the voters in western PA who have abandoned the Democrats in presidential and other elections are actually still members of the Democratic Party.

The following is a map with all counties where registered Democrats outnumber registered Republicans colored in blue:

In 2012, that map held up almost perfectly in eastern PA but in western PA every one except Allegheny and Erie switched over to the GOP.

It had been my assumption, and the belief of others, that this trend would continue with Hillary Clinton as the nominee. But has it?

For example, in last April’s primary Clinton outperformed Trump in eight counties, including Beaver and Fayette (Sanders finished with more votes than Clinton and Trump in Centre County, home of Penn State).

I found this a bit surprising. Those counties are supposed to be the white working class Rust Belt areas that Trump is depending on. In fact, MSNBC recently did a segment on this phenomenon in Beaver County of all places.

Another assumption was that the white working class Appalachian voters that flocked to Hillary in the 2008 Democratic primary had switched over to Bernie. They did in West Virginia and Kentucky, but in Pennsylvania and Ohio they did not.

What could account for this?

All four states held primaries and Clinton competed in every one of them except West Virginia. Perhaps some event intervened between the time Ohio and PA voted (March 15th and April 26th respectively) and the time West Virginia and Kentucky voted (May 3th and May 10th respectively)?

You’ll recall for instance that Clinton’s comment about putting coal miners “out of business” garnered a lot of attention and was considered a big issue in these contests. But Clinton said that on March 14th in Ohio and it didn’t hurt her there or in Pennsylvania over a month later.

It could be Clinton supporters returning, as Bill Clinton won many western PA counties in 1992 and 1996. If that were the case, though, then why didn’t this occur in West Virginia and eastern Kentucky. Additionally, as hard as it is to hear, 1996 was a long time ago. Babies born that year votd in last month’s primary for instance.

Maybe western PA is more steel than coal and more Rust Belt than Appalachia, and perhaps that is a subtle yet key difference.

Ultimately, it is still early in the general election process and primary results are often not a great indicator of what will happen in November. Nevertheless, if Clinton performs better than Barack Obama in western PA it would be extremely difficult for Donald Trump to pry away the Keystone State and its essential 20 electoral votes.