WASHINGTON — US Vice President Kamala Harris's election eve schedule includes four appearances in the state, ending with rallies in Pittsburgh and Philadelphia featuring a slate of celebrity guests, including Oprah Winfrey.

Former President Donald Trump is also barnstorming the must-win state, holding two rallies in Pittsburgh and Reading.

According to calculations by elections analyst Nate Silver, the candidate who wins Pennsylvania has more than a 90% chance of winning the White House.

“It’s the granddaddy of all the swing states,” said former congressman Patrick Murphy, who represented north-eastern Pennsylvania as a Democrat from 2007-11.

With its 19 electoral votes, Pennsylvania – the fifth most populous US state— is the lynchpin of the swing-state electoral firewalls for both Kamala Harris and Donald Trump.

If the Democrats win Pennsylvania, Wisconsin and Michigan, along with one congressional district in Nebraska, she’s the next president. If the Republicans carry Pennsylvania, North Carolina and Georgia, Trump is back in the White House next year.

Without Pennsylvania, there is no way Trump can win without flipping at least three of the states Joe Biden won in 2020.

Nicknamed the Keystone State, Pennsylvania could in fact be the key to the White House.

Pennsylvania is not only the most valuable swing state, it also can be seen as a microcosm of the US as a whole – demographically, economically and politically.

It is a former manufacturing state that has been transitioning to newer industries and businesses, but it has a large energy sector because of its abundant oil shale deposits. Agriculture is still the second-largest industry in the state.

The majority of the population is white, but there are growing immigrant communities. Some areas, like Allentown – the working-class factory city made famous by a Billy Joel song – are now majority Hispanic. The state’s black population, at 12%, is just under the US total of 13%.

As for the politics, the state’s two large urban areas, Philadelphia and Pittsburgh, heavily favour the Democrats. Between the two are vast stretches of rural territory where Republicans dominate. And the suburbs that once were reliably conservative are now tilting to the left.

That gives rise to the old quip that Pennsylvania is Philadelphia and Pittsburgh with (deeply Republican) Alabama in the middle.

Somehow, all these political cross-currents and shifting dynamics have kept Pennsylvania at a near dead-even balance when it comes to presidential elections. President Joe Biden won the state by about 80,000 votes in 2020. Donald Trump carried it by about 40,000 in his surprise 2016 win over Hillary Clinton.

Only once in the last 40 years has a candidate won Pennsylvania by double-digits— Barack Obama in his 2008 electoral landslide.

The final New York Times/Siena College poll of the state— published on Sunday— suggested the two were locked in a tie, with both Trump and Harris at 48% of likely voters. Before then, they were locked in a virtual dead heat, with Harris slightly ahead.

Both the Harris and Trump campaigns have been pouring enormous resources into Pennsylvania. They are spending more on television advertising there than any other swing state and made regular visits even before the campaigning frenzy on election eve.

Harris introduced her running mate pick, Tim Walz, at a rally in Philadelphia. She spent days preparing for her presidential debate in Pittsburgh. She made a tentpole economic speech in September.

In early October, Trump held a massive rally in Butler, where in July he was nearly assassinated. He was in Biden's hometown of Scranton and then Reading just a couple of weeks later.

And when the principals aren’t around, both campaigns have other politicians and officials to drum up support.

“A candidate can't go into a county to talk to 1,200 people,” says former Democratic Pennsylvania Governor Ed Rendell. “The state is too big. There’s just not time. That’s what surrogates are for.”

Rendell notes that the current governor, Democrat Josh Shapiro, is a big help for Democrats here, as he is very popular in the state and a dynamic speaker – qualities that had made him the odds-on favourite to be Harris’s vice-presidential pick.

For Harris, her keys to victory are to post dominating numbers in Philadelphia and Pittsburgh and win the suburbs by enough to offset Trump’s margins in the rest of the state.

An essential part of this strategy is to win over moderate voters and some Republicans – including the more than 160,000 who turned out to vote for former South Carolina Governor Nikki Haley in the state’s Republican primary, held earlier this year, well after Trump had already locked up the party’s nomination.

“What these people need to hear is ways in which both the past record of Kamala Harris and the future plans of Kamala Harris are basically centrist positions – that she is not this crazy, wild-eyed radical leftist,” said Craig Snyder, former Republican Senate staffer who is running Pennsylvania's “Haley Voters for Harris” effort.

He added that the Harris campaign is making the most extensive effort to reach Republican voters that he’s seen in a generation. — BBC