The rally saw thousands of Trump supporters snaking through an exposed field and parking lot under baking conditions, gathering in Harrisburg's New Holland indoor arena, which is named for a tractor brand.
"I want democracy and that's what Trump represents... if we had democracy, we would have had a (Democratic Party) primary and Kamala would not have just been picked out of nowhere," said Debbie Brown, a Trump supporter in her 50s.
"He will beat her if it's a fair election (even) with Hollywood standing behind her because that's all they're about -- elites, and making sure the elites are taken care of. They don't care about the American people."
Under pressure after a disastrous debate performance, US President Joe Biden, 81, dropped out of the presidential race on July 21, backing Vice President Harris as his replacement. The Democratic party's leadership has supported the switch.
- Pejorative attacks -
The change in candidate has left Trump's campaign struggling to switch gears, with the former president trying various lines of pejorative attacks against her.
Trump has also stepped up campaigning in the important swing state of Pennsylvania, where polling has shown Harris is closing the gap on her right-wing rival.
On Wednesday, at an event in Chicago, he questioned Harris's racial identity, accusing her of opting to "turn Black."
Harris is the first Black, woman, and South-Asian-heritage vice president in US history.
At the rally in Harrisburg, however, Trump's message was being echoed.
"She's Indian-Jamaican. And so they're trying to package her as an African-American, but she doesn't have that heritage of folks who were brought here by slavery," said Sean Moon, 45, a prominent right-wing pastor.
"She's not really representing that whole culture. They were trying to package her that way."
- 'Fight, fight, fight' -
"She's gonna support all the radical leftist positions that Christians should be against," added Moon, who is from Tennessee and wore gold Trump-brand "Never Surrender" high-top sneakers.
Nearby a merchandise seller chanted "fight, fight, fight, you missed bitches," a reference to Trump's narrow escape from an assassin's bullet in Butler, Pennsylvania, at a July 13 rally.
Since then, Trump has stopped holding outdoor rallies, although he has vowed to return to Butler to hold another campaign event.
Other Trump supporters attacked Harris on crime, another key Republican talking point heading into the election.
"I don't like her -- she's a California liberal. She's anti-law enforcement," said James Subach, 59, of the former prosecutor. "I'm a former cop, and you've got to have rules."
Subach called on Trump to "trash the green energy stuff."
"(That's) not going to fly here in Pennsylvania, we were rich in coal industry here at one time but that has been suppressed by the Democrats," he said.
"Trump's a businessman and can straighten out this country."
- 'Not going to accept' -
Several rally-goers raised fears that the Harris campaign might seek to steal November's election, echoing unfounded claims Trump has made following his defeat by Biden in 2020.
"For example, all the people coming across the border right now. You've got all these sanctuary cities already registering these people to vote," said 40-year-old tattoo artist Edwin Black, invoking a falsehood that has gained traction online.
"(Harris) hasn't done anything in four years... once a cheater, always a cheater."
Black suggested that people were "not going to accept" a Harris victory, even if it was announced officially.
"Now this time more, more than enough people have woken up, so it's not going to fly," he said.
