In the final sprint of the presidential campaign, both the Biden and Trump camps set their eyes on Pennsylvania, aptly referred to as the Keystone State, a crucial contest that may determine the next president of the United States.

President Trump rallied supporters Monday in Wilkes-Barre two days after holding four separate events across Pennsylvania. Vice President Pence campaigned Monday in Latrobe and in Erie, a county that swung from Obama to Trump in 2016.

The Biden campaign also carpeted the state Monday in a series of stops that reflect its diversity: in Beaver County with union members, in Pittsburgh with African American community leaders and at an event for Latino voters in Lehigh Valley. Jill Biden appeared in Erie as well and attended a rural get-out-the-vote event in Lawrence County.

Both campaigns have emphasized how crucial Pennsylvania is to their electoral prospects. In 2016, Trump flipped the state, winning by less than 1 percentage point. Before that year, Pennsylvania had voted for Democratic candidates in six consecutive presidential elections.

Speaking Sunday in Philadelphia, Biden said: "My message is simple, Pennsylvania is critical in this election."

Trump told supporters in Allentown on Oct. 26, "By the way, we win Pennsylvania, we win the whole thing."

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How did Trump flip Pennsylvania?

Pennsylvania is geographically diverse, with a history of blue-collar workers in the coal and steel industries (some of whom have transitioned to working in the natural gas industry), ample farmland and a large health care sector.

According to the Center for American Progress, 84% of Pennsylvania voters in 2016 were white and 54% of the state's total vote came from non-college-educated white voters. The turnout rate among these voters increased from 53% in 2012 to 57% in 2016. Trump was able to boost turnout both among white and rural voters.

"Our estimates show that if white non-college-educated turnout had remained at its 2012 level, instead of increasing significantly as it did, [Hillary] Clinton would have been able to carry the state," a Center for American Progress report from 2017 said.

How many electoral votes?20

Are people voting by mail?Yes. According to the U.S. Elections Project, a turnout-tracking database run by University of Florida professor Michael McDonald, more than 3 million mail-in ballots were requested in the state.

As of Monday, more than 2.4 million ballots have been returned, about 10 times the number of mail-in ballots used in 2016, Pennsylvania Secretary of State Kathy Boockvar told NPR's Morning Edition on Tuesday.

McDonald noted that registered Democrats in the state have a roughly 1.2 million ballot request lead over registered Republicans.

More than 95 million Americans across the country have voted early.

What's the state of the race?

Pennsylvania, which NPR regards as a "Lean Democratic" state in the election, is one of the most hotly contested states this cycle.

Trump appears to be more competitive in Pennsylvania than in two other swing states that he won in 2016 — Michigan and Wisconsin.

While Biden had a 7-point lead in Pennsylvania three weeks ago, the race has tightened in recent days. As NPR's Domenico Montanaro reported, the narrowing of the race is something the Biden campaign was expecting in the final days of the campaign.

By NPR's count, if Trump wins Georgia, Iowa, Florida, North Carolina, Arizona and Maine's 2nd Congressional District, and Biden retains the support in states that are now leaning his way, the race could become a 259-259 electoral showdown with Pennsylvania as the deciding state.

This time, the fight for Pennsylvania, and the presidency, may hinge on who wins the suburbs and by how much — specifically in the southeast counties of Bucks, Delaware, Chester and Montgomery. Polls show that the president has lost support among these voters — specifically suburban women.

Trump acknowledged as much during an Oct. 13 campaign stop in the southwest city of Johnstown when he said: "Suburban women, will you please like me? Please, please."

Trump has pushed a "law and order" message, hoping to peel off more support from suburban women. He's also repeated claims that Biden will decimate the state's fossil fuel industry.

Biden, on the other hand, has made Trump's handling of the coronavirus pandemic a central issue of his campaign and has pushed the message that the Trump administration is trying to dismantle the Affordable Care Act in the middle of this crisis. He's also emphasized his Scranton roots during campaign events in the state. The Democrat was born in the state but moved with his family to Delaware as a boy.

Obama has campaigned in the state as well, with a particular focus on boosting turnout among Black men.

When will we know Pennsylvania's results?

Pennsylvania election officials are cautioning the public that the state's full results won't likely be known on election night.

Boockvar, the secretary of state, told NPR that the overwhelming number of ballots will be counted "within a couple days." The state has up to 20 days to certify the election.

But on Thursday, Gov. Tom Wolf put it more bluntly: "We're sure it will take more time than it used to. We probably won't know results on election night."

Pennsylvania Republicans had previously tried to block counting ballots arriving after Election Day. But the U.S. Supreme Court has declined to overturn the state Supreme Court's decision that election officials can count absentee ballots received as late as the Friday after Election Day so long as they are postmarked by Nov. 3.

What's the counting process like?

Pennsylvania and Wisconsin are the only two swing states where election officials can only begin processing and counting absentee ballots on Election Day, which means complete results are likely to be delayed for several days.

Many counties said they'll start processing ballots as soon as they're allowed: early Tuesday morning. But as NPR's Sam Gringlas reported, a handful of counties, such as Cumberland County outside Harrisburg, said they won't begin dealing with absentee ballots until Wednesday.

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