The fourth in a 10-part weekly series

ALTOONA, Pa. – Donald Trump campaign signs decorate the lawns along state Route 26, spots of blue peeping out in front of manufactured home after manufactured home, just a step up from the trailer park down the street.

This is Trump country – coal country -- where for-sale signs and going-out-of-business signs join the ones for the GOP nominee.

The economy hasn’t recovered here as much as in other parts of the nation. And coal is a decreasing business in Pennsylvania, thanks in part to tougher restrictions from the Obama administration.

Trump has vowed to bring it back.

“We’re going to save the coal industry and other industries threatened by Hillary Clinton’s extremist agenda,” he said in May.

The businessman’s tough talk appeals to voters in the state’s central and southwest regions – home of the declining steel industry -- where the population is mostly white and the recreational activities mainly are hunting and fishing.

And it’s the support from these white working-class voters that has the GOP hoping Trump can carry the Keystone State for the party – the first time in 28 years.

But 200 or so miles from here is Philadelphia, which is Hillary Clinton country and Barack Obama country.

The City of Brotherly Love, where America declared its independence and where many young voters and minority voters make their homes – this is key for Democrats, as are other urban areas like Pittsburgh, Harrisburg, and State College.

These are the two Pennsylvanias that political candidates have to navigate in order to win the state.

“Essentially Pennsylvania is two different states. Philadelphia and its suburbs – if it was its own state – would be a solid blue state and would be uncompetitive, and then the rest of the state would be solid red and be uncompetitive,” said Patrick Murray, director of the Monmouth University Polling Institute. “You combine them together and it’s a combination of turnout and the base.”

Turning out their base while appealing to the state’s small segment of swing voters is what Trump and Clinton have to balance as they fight it out here.

On its surface, Pennsylvania looks like a red state – literally. Viewing a map of the 2012 presidential results, 90 percent of the state is red with just a few patches of blue.

But it’s where the blue is located that counts. Highly populated urban areas like Philadelphia, Pittsburgh and Harrisburg make up about a fourth of total registered Democrats in the state.

That has made Pennsylvania reliably blue for Democrats, going for the party in every presidential election since 1992. And they want to keep it that way. Democrats held their nominating convention in Philadelphia in July and, after accepting the nod, Clinton and running mate Tim Kaine embarked on a bus tour that took them through Pennsylvania and into Ohio.

The party has an almost 1 million voter advantage in registered voters statewide – 4.1 million to the GOP’s 3.2 million.

And Clinton is leading by 6.6 percent in the RealClearPolitics Polling Average.

But Republicans do well here in midterm elections. The GOP controls both houses in the state General Assembly, holds one of the Senate seats and 13 of 18 U.S. House seats.

“It’s all about the Philadelphia suburbs. That’s where the largest pool of swing voters live.” G. Terry Madonna, director of the Franklin & Marshall College Poll

This fact and that large swath of red on the map are what make Pennsylvania a battleground state – if Trump can take its 20 electoral votes, it could signal wins in nearby swing states of Ohio, Michigan, and Wisconsin. And while there are many paths to the 270 electoral votes needed to win the presidency, it’s hard for either side to get there without Pennsylvania.

And the key to tipping the state toward one party or another are the four counties that surround Philadelphia: Bucks, Chester, Delaware, and Montgomery.

“It’s all about the Philadelphia suburbs. That’s where the largest pool of swing voters live, and if you don’t win the Philly suburbs, it’s virtually impossible to win the state, simply because of the numbers,” said G. Terry Madonna, director of the Franklin & Marshall College Poll.

Obama won the state with 52 percent of vote in 2012 with 10 counties out of 67, but he had the key areas: Philadelphia, Pittsburgh, Harrisburg, and three of those four swing counties (he lost Chester by one percentage point).

And to reiterate how important this small, southeastern corner of the state is to victory: In 2012, almost 2 million votes, or about one-third of all the votes cast in the state, came from Philadelphia and Bucks, Chester, Delaware, and Montgomery counties.

The candidates know this. And they know the dual strategy needed to win here: campaign in the parts of the state that will bring out your base combined with courting voters in those collar counties ringing Philadelphia.

The voters in the four key counties tend to be college-educated, with more women than men. There is a higher minority population there than in the rest of the state (with the exception of urban areas like Philadelphia and Pittsburgh). The voters in these collar counties are fiscally conservative but socially liberal – for gay marriage and gun control. Democrats have a slight advantage in voter registration in these four but it’s only by about 53,000 votes. And the counties have a high percentage, compared to the rest of the state, of residents and families in the $100,000-or-higher income bracket, according to the Pennsylvania State Data Center.

Trump was in Aston, Pa., earlier this month – part of Delaware County – to tout his child-care plan and he brought along one of his most accomplished surrogates to help appeal to the area’s female voters.

“As I travel around the county with my father, stories about the hardships caused by our existing child-care system -- one that is too expensive, too outdated, and too inaccessible -- come up time and time again,” Ivanka Trump said.

State GOP Chairman Rob Gleason concedes it won’t be easy to win the state. For Trump, it’s a question of turning out as many voters as he can in the red rural areas while courting the swing counties and hoping the Democratic base fails to turn out in the urban areas.

“What he needs to do is over-perform across the state. It has happened in the past. Sometimes the Democratic candidate does not perform well in the cities – they always win,” Gleason told RealClearPolitics, “but it all depends on how well that Democratic candidate will perform in the cities.”

“What [Trump] needs to do is over-perform across the state. It has happened in the past.” State GOP Chairman Rob Gleason

And Clinton has brought in the big guns to help with that turnout: President Obama and Sen. Elizabeth Warren in Philadelphia, Vice President Biden in his hometown of Scranton, Chelsea Clinton in State College, and former President Bill Clinton in Pittsburgh. First lady Michelle Obama will be in the Philadelphia and Pittsburgh areas next week.

The reason is simple: One thing that could put Pennsylvania in the red this year is the lack of enthusiasm among Democrats for Clinton.

“It’s back to the enthusiasm gap,” Madonna said. “And the millennials don’t look like at the moment they’re going to vote for Clinton the way they voted for Obama. So it’s a combination of her base – are they enthusiastic? – and the other question is what can Trump actually do in terms of the oomph.”

President Obama addressed the enthusiasm issue in his rally at the Philadelphia Art Museum last week, appealing to Democrats, and particularly millennials, to support Clinton.

“The time has come for me to pass the baton on, but I know Hillary is going to take it and she’s going to run that race and she will finish that race and that’s why I’m with her. That’s why I’m fired up. That’s why I’m ready to go. And I need you to join me. I need you to work as hard for Hillary as you did for me,” he told the crowd.

Clinton herself appealed to millennials during a speech Monday at Temple University in Philadelphia. She invoked Bernie Sanders’ name in her pitch – the Vermont senator remains popular among younger voters.

“We came up with a plan that makes public college tuition-free for working families and debt-free for everyone,” she said. “And if you already have debt, we will help you refinance it and pay it back as a percentage of your income so you’re never on the hook for more than you can afford.”

Democratic Party state Chairman Marcel Groen said that in addition to turning out their base, they are also courting the youth vote.

“We need to reach [millennials] to make sure they come out and vote. And we’ll do that by a variety things including simply door knocking.” State Democratic Party Chairman Marcel Groen

“Less and less of them watch television. They watch everything on computer, they stream everything. They’re hard to reach. But when you reach them you find out their values are good,” he told RCP. “We need to reach them to make sure they come out and vote. And we’ll do that by a variety things including simply door knocking.”

And the Democrats also see themselves getting some help from the GOP, thanks to Trump’s tendency to stumble with voters with his controversial remarks.

“I think our best turnout [motivator] is the Republican candidate and his continuously insensitive and insulting remarks,” Groen said.

And, for Clinton, Pennsylvania is personal. Neither she nor Bill has ever lost in the state. Hillary Clinton’s win here in the April 2008 presidential primary kept her bid against Obama alive for a few more months.

But Gleason, the GOP chairman, dismissed Clinton’s 2008 win as more of an anti-Obama vote.

“The reason for that was there’s a lot of prejudice in Pennsylvania. And there were a lot of people who were not going to vote for a black person in many of the counties in Pennsylvania and that really fueled her campaign. That’s the honest assessment. Some people won’t want to tell you but that’s true,” he said.

All this means Pennsylvania voters are going to be inundated until Election Day with campaign visits, ads, and get-out-the-vote efforts from both parties.

“We’re going to win Pennsylvania,” Groen said. “We just can’t take it for granted. We have to bust our ass and work hard.”

Next week: Iowa

Emily Goodin is the managing editor of RealClearPolitics.