Focusing on Pennsylvania's southeast corner, Trump took the stage in Reading, about 30 miles from Allentown, where Harris held her own event about half an hour later.
"If we win Pennsylvania, we win the whole ball of wax," Trump said. "It's over."
Indeed, a Trump victory in Pennsylvania, flipping its 19 Electoral College votes, would puncture the Democrats' "blue wall" and make it harder for Harris to win the necessary 270 votes.
Live election 2024 updates in the Philadelphia region and the focus on Pennsylvania
Harris, the Democratic nominee, spent all of Monday in Pennsylvania, the largest prize among the states expected to determine the Electoral College outcome, and offered a similarly blunt assessment.
"We need everyone in Pennsylvania to vote," she said. "You are going to make the difference in this election."
In addition to Allentown, Harris visited Scranton - the birthplace of President Joe Biden - and had a stop planned in Reading before ending with a late-night Philadelphia rally that was to include Lady Gaga and Oprah Winfrey.
"Are you ready to do this?" Harris yelled Monday in Scranton, with a large handmade "VOTE FOR FREEDOM" sign behind her and a similar "VOTE" banner to her side.
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Trump went first to North Carolina before visiting Reading. He will head to Pittsburgh, at the opposite end of the state, before concluding in Grand Rapids, Michigan, where he will hold his last campaign rally in the same place he concluded his 2016 and 2020 runs.
Southeast Pennsylvania is home to thousands of Latinos, including a sizable Puerto Rican population. Harris and her allies have repeatedly hit Trump for a comedian's dig at Puerto Rico during the former president's marquee Madison Square Garden event. The comedian, Tony Hinchcliffe, referred to Puerto Rico as a "floating island of garbage."
"It was absurd," said German Vega, a Dominican American who lives in Reading and became a U.S. citizen in 2015. "It bothered so many people - even many Republicans. It wasn't right, and I feel that Trump should have apologized to Latinos."
But Emilio Feliciano, 43, waited outside Reading's Santander Arena for a chance to take a photo of Trump's motorcade. He dismissed the comments about Puerto Rico despite his family being Puerto Rican, saying he cares about the economy and that's why he will vote for Trump.
"Is the border going to be safe? Are you going to keep crime down? That's what I care about," he said.
Harris told the crowd, "I stand here proud of my long-standing commitment to Puerto Rico and her people."
"And I will be a president for all Americans," she said, adding that "momentum is on our side. Can you feel it?"
Trump, meanwhile, stuck to talking about his proposed crackdown on immigration. He called to the stage Patty Morin, the mother of 37-year-old Rachel Morin, who was found dead a day after she went missing during a trip to go hiking. Officials say the suspect in her death, Victor Antonio Martinez Hernandez, entered the U.S. illegally after allegedly killing a woman in his home country of El Salvador.
About 77 million Americans have voted early. A victory by either side would be unprecedented.
Trump winning would make him the first incoming president to have been indicted and convicted of a felony, after his hush-money trial in New York. He will gain the power to end other federal investigations pending against him. Trump would also become only the second president in history to win nonconsecutive White House terms, after Grover Cleveland in the late 19th century.
Harris is vying to become the first woman, first Black woman and first person of South Asian descent to reach the Oval Office - four years after she broke the same barriers in national office by becoming President Joe Biden's second in command.
The vice president ascended to the top of the Democratic ticket after Biden's disastrous performance in a June debate set into motion his withdrawal from the race - one of a series of convulsions that hit this year's campaign.
Trump survived by millimeters an assassination attempt at a rally in Butler, Pennsylvania. His Secret Service detail foiled a second attempt in September, when a gunman had set up a rifle as Trump golfed at one of his courses in Florida.
Harris, 60, has pitched herself as a generational change from 81-year-old Biden and Trump, who is 78. She's emphasized her support for abortion rights after the 2022 Supreme Court decision that ended the constitutional right to abortion services, and she has regularly noted the former president's role in the Jan. 6 attack on the U.S. Capitol.
Assembling a coalition ranging from progressives such as Rep. Alexandria Ocasio-Cortez of New York to Republican former Vice President Dick Cheney, Harris has called Trump a threat to democracy and late in the campaign even embraced the critique that Trump is accurately described as a " fascist."
Heading into Monday, Harris has mostly stopped mentioning Trump by name, calling him instead "the other guy." She is promising to solve problems and seek consensus.
Harris campaign chair Jen O'Malley Dillion said on a call with reporters that not saying Trump's name was deliberate because voters "want to see in their leader an optimistic, hopeful, patriotic vision for the future."
Harris also offered some insights into her personal formation as a politician that she doesn't often divulge. In Scranton, she talked about once being a longshot while running for San Francisco district attorney in 2002 and how she "used to campaign with my ironing board."
"I'd walk to the front of the grocery store, outside, and I would stand up my ironing board because, you see, an ironing board makes a really great standing desk," the vice president said, recalling how she would tape her posters to the outside of the board, fill the top with flyers and "require people to talk to me as they walked in and out."
In Allentown, Harris rallied with rapper Fat Joe, and she planned to visit a Puerto Rican restaurant in Reading with leading progressive voice New York Rep. Alexandria Ocasio-Cortez. Both Fat Joe, whose real name is Joseph Cartagena, and Ocasio-Cortez, are of Puerto Rican heritage.
Standing in line for Harris' Allentown rally, 54-year-old Ron Kessler, an Air Force veteran and Republican-turned-Democrat, said he planned to vote for just the second time in his life. Kessler said that, for a long time, he didn't vote, thinking the country "would vote for the correct candidate."
But "now that I'm older and much more wiser, I believe it's important, it's my civic duty. And it's important that I vote for myself and I vote for the democracy and the country."
As recently as Sunday, Trump renewed his false claims that U.S. elections are rigged against him, mused about violence against journalists and said he " shouldn't have left" the White House in 2021 - dark turns that have overshadowed another anchor of his closing argument: "Kamala broke it. I will fix it."
The Associated Press contributed to this report.