Former President Donald Trump on Monday said Vice President Kamala Harris would turn the U.S. into a “de-industrialized third-world nation” if she wins the presidency.

At a campaign stop at a machining plant in York, Pennsylvania, Mr. Trump described the high stakes in the presidential election.

He said a Harris administration would rob Pennsylvania of key energy jobs and treat thieves and criminals lightly, hoping to undercut Ms. Harris’s claim she is a former prosecutor who can ensure public safety.

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“Pennsylvania is gonna tell her we’ve had enough,” Mr. Trump said at Precision Custom Components. “Comrade Kamala Harris, you’re fired.”

Mr. Trump said Ms. Harris would decimate Pennsylvanian energy jobs, pointing to her 2019 opposition to fracking. Her campaign has backed off that position.

“On Day One, I will tell the frackers and energy workers of Pennsylvania to drill, baby, drill,” Mr. Trump said. “She was a non-fracker. We will embrace all forms of energy, including nuclear.”

The event had a more subdued feel than Mr. Trump’s rallies. The ex-president spoke in a milder tone and stood in front of factory equipment and business leaders instead of an adoring throng of supporters

Pennsylvania is a must-win state for both Mr. Trump and Ms. Harris. It will hand out 19 electoral votes in November, making it a major swing-state prize.

Mr. Trump won the state in 2016 but President Biden put it back in the Democratic column in 2020.

Polls show them neck-and-neck in Pennsylvania, where Ms. Harris has erased Mr. Trump’s roughly 4-point lead over President Biden before he dropped out of the race.

Mr. Trump is trying to dim the glow of the Democratic National Convention in Chicago this week by barnstorming the nation with campaign events. 

Each one of them has a theme. Monday’s event focused on the economy and energy. He will follow that up with a crime-focused event Wednesday in Michigan, a national security event in North Carolina, and an immigration event Thursday at Montezuma Pass in Arizona.

He will close out the week with a rally on Friday in Glendale, Arizona, and a “no tax on tips” event in Las Vegas.

His running mate, Sen. J.D. Vance of Ohio, also has a slate of planned events that will take him to Philadelphia, Wisconsin, North Carolina and Georgia. 

Mr. Trump said he handed Mr. Biden and Ms. Harris a surging economy, glossing over the COVID-19 crisis that upended the economy and supply chains during Mr. Trump’s last year before inflation surged under the Biden administration.

“They quickly blew it all up,” Mr. Trump said.

He also said Ms. Harris cast the tie-breaking votes in a divided Senate that fueled inflation through government spending. 

Some economists say the federal spending overheated the economy even as supply chains struggled to produce goods. Mr. Biden and Ms. Harris say the inflation problem is easing and that companies are engaging in price gouging even though their operations are back to normal.