Harris, Trump battle for the biggest of the battlegrounds: 'It's clearly ground zero'

With a margin-of-error race and just three weeks to go until Election Day, Vice President Kamala Harris and former President Trump on Monday each campaigned in the biggest of the battleground states.

But the Democratic and Republican presidential nominees will be on the opposite ends of Pennsylvania, with Harris holding events in Erie, in the northwest corner of the state, and Trump in suburban Philadelphia, in the southeastern portion of the commonwealth.

It's the vice president's 10th visit to Pennsylvania since replacing President Biden atop the Democrats' 2024 ticket in mid-July. Trump's also been a frequent visitor, with rallies in Scranton and Reading less than a week ago.

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Vice President Kamala Harris and former President Trump debate in Philadelphia on Sept. 10, 2024. (Doug Mills/The New York Times/Bloomberg via Getty Images)

Pennsylvania, along with Michigan, Wisconsin, Georgia, North Carolina, Arizona and Nevada, had razor-thin margins that decided Biden's 2020 victory over Trump. And the seven states will likely determine whether Harris or Trump wins the 2024 presidential election.

But with 19 electoral votes at stake, Pennsylvania's the biggest of the key battlegrounds. And while the campaigns and their allied super PACs are pouring resources into all seven states, more money has been spent to run spots in Pennsylvania than any of the other battlegrounds, according to figures from AdImpact, a top national ad tracking firm.

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Harris starts her latest stop in the Keystone State with a conversation with Erie area Black men at a locally owned small business, as part of her efforts to court a group of voters traditionally loyal to the Democratic Party.

The vice president will then headline a rally at the Erie Insurance Arena. Democratic Sen. John Fetterman is also scheduled to speak.

Vice President Kamala Harris addresses the Economic Club of Pittsburgh on the Carnegie Mellon University campus, Sept. 25, 2024. (AP Photo/Gene J. Puskar)

"At the rally, the Vice President will highlight her vision for a New Way Forward, while encouraging Pennsylvanians to vote early or return their mail ballot," the Harris campaign said.

Erie is a Democratic-dominated city of roughly 94,000 people that's surrounded by suburban and rural areas with significant numbers of Republican voters. Erie County has long been considered one of the state's handful of bellwether counties. 

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Trump, who was last in Erie on Sept. 29, is scheduled to hold a town hall at the Greater Philadelphia Expo Center and Fairgrounds in Oaks, in suburban Philadelphia.

Former President Trump speaks at a campaign rally at Riverfront Sports, Wednesday, Oct. 9, 2024, in Scranton, Pennsylvania. (AP Photo/Julia Demaree Nikhinson)

Pennsylvania, along with Michigan and Wisconsin, are the three Rust Belt states that make up the Democrats' so-called "Blue Wall."

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The party reliably won all three states for a quarter-century before Trump narrowly captured them in the 2016 election to win the White House.

Four years later, in 2020, Biden carried all three states by razor-thin margins to put them back in the Democrats' column and defeated Trump.

Mark Harris, a Pittsburgh-based longtime Republican national strategist and ad maker, told Fox News that Pennsylvania is "the one state that it's hard to see someone losing and then still winning the presidential race."

Harris, a veteran of multiple GOP presidential campaigns, emphasized that "it's clearly ground zero."

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.