Pennsylvanians like to think we’re a general election swing state. We’re really not.

Since 1988, Pennsylvania has been reliably blue and though some Republicans have done well to win a statewide race in non-presidential election years, Democrats hold a registration advantage of nearly a million people. If the Democrats turn out — which they traditionally do in higher numbers in presidential election years — the math says the candidate for the White House will win the Keystone State.

But when broken down by party, at least in this year’s topsy-turvy race we’re calling our presidential primary, Pennsylvania stands to actually matter in both the Republican and Democrat presidential primaries and holds one of the largest keys to which party will control the Senate by the end of 2016 — which could have a ripple effect to the Supreme Court.

Many political insiders have said over the years that the secret to winning Pennsylvania is cleaning up in Philadelphia and its suburbs. We wanted to break down voter registration trends to see if that holds up and show how the Philadelphia suburbs’ slow creep toward the left has changed the entire state of Pennsylvania and how it votes.

Republicans vs. Democrats

The GOP is on a six-election losing streak in Pennsylvania presidential elections, but Republicans have won statewide office in recent years. Sen. Pat Toomey, up for re-election this year, is a Republican and this state has elected as many Republican governors as it has Democrats. Thirteen of the state’s 18 congressmen (yeah, they’re all men) are Republicans.

Yet Democrats hold a huge voter registration majority. So what gives?

In short, it comes down to turnout. Democrats typically perform better when voter turnout is higher (read: presidential election years) and trends from the Department of State show more inactive voters on the left than the right.

Let’s look first at the general election. Democrats will win Pittsburgh and Philadelphia by landslide margins. In Pittsburgh, the Democrat to Republican registration is nearly 3-to-1. In Philly, it’s almost 8-to-1. Meanwhile, Republicans will carry the “T-region” of Pennsylvania. These are the rural areas that fall along the state if you drew a giant capital T on it.

Where candidates stand to win or lose Pennsylvania is in the Philadelphia suburbs.

A third of the 8.2 million registered voters in the state are in Philadelphia and the four counties that surround it (Bucks, Chester, Delaware and Montgomery). More than 38 percent of all registered Democrats in Pennsylvania live in one of those five counties while a quarter of all registered Republicans live in the area. Any candidate who wins those five counties will be tough to beat.

The Philadelphia suburbs were at one point on the redder side, but in the last decade have gained more and more Democrats. Here’s what the current voter registration statistics look like:

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Philadelphia and its suburbs represent more voters than the next six counties with the highest voter registration combined and each of those counties includes the other largest cities in the state, including Pittsburgh and Allentown where Democrats also enjoy higher voter registration.

This is why you’re seeing presidential candidates in places like Philly, Delaware County, Montgomery County, Pittsburgh and the Lehigh Valley because both sides of the aisle are thinking about the general and well know that if the Democrats carry the Philly suburbs in addition to the state’s two largest cities, that lead would be very difficult to overcome by a Republican candidate.

And the GOP’s decreasing hold of the area is one of the largest reasons why the right hasn’t won a presidential general election in Pennsylvania since George H. W. Bush.

The upcoming primary

Because Pennsylvania operates a closed primary in which independents can’t cast ballots, Philadelphia and its suburbs are still vital to winning each primary. What makes the four counties surrounding Philly unique is that they’re voter-rich, but they’re almost split down the middle Democrat to Republican. In a primary situation, that split matters less.

Here’s that same graph you saw earlier, but flipped to show how the registration of each party differs between the five counties in question:

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Where Pennsylvania will really come into play in the presidential primary — and, by extension, the primaries for Senate, state attorney general and local races — is voter turnout. The number of people who come to vote in presidential primaries is typically significantly lower than in general elections. In the 2008 presidential primary, 42.7 percent of registered voters cast ballots.

But this year is different. Voters are registering Republican in droves to vote for Donald Trump (or maybe against him) and thousands of independents are switching to Democrat — some say in order to vote for Vermont Sen. Bernie Sanders in the primary. In addition, more than 174,000 have used the state’s new online voter registration to become newly-registered voters in just the 2016 calendar year.

Political observers are expecting turnout to far exceed the 2014 gubernatorial primary. Still, the ‘burbs hold the key to the general.

“Democrats will win Philadelphia and Pittsburgh overwhelmingly, Republicans will carry the rural areas in central Pennsylvania and do well in the western towns outside of Pittsburgh,” a Republican strategist wrote in Real Clear Politics, “but the huge shift that has made Pennsylvania reliably Democratic in presidential years has been the change in voting patterns in the Philadelphia suburbs.”

Anna Orso was a reporter/curator at Billy Penn from 2014 to 2017.