Former President Donald J. Trump’s words thundered from screens at a packed campaign rally on Monday night in Erie, Pa. But the event was for Vice President Kamala Harris, who was using Mr. Trump’s own words as her campaign amplified warnings of the dangers she says he poses should he win a second term in the White House.

Ms. Harris pulled few punches as she portrayed her Republican opponent as an authoritarian obsessed with his own power, pointing to Mr. Trump’s recent rallies and media appearances where he has asserted that his Democratic detractors were the “enemy from within,” more dangerous than foreign adversaries like Russia and China, and that they “should be put in jail.”

“After all these years, we know who Donald Trump is,” Ms. Harris said. “He is someone who will stop at nothing to claim power for himself.”

In a striking moment, Ms. Harris told the crowd of 6,000 that they didn’t have to take her word for it, that she had an example of his “worldview and intentions.”

“Please — roll the clip,” she said as the crowd groaned and gasped as Mr. Trump’s face flashed on screens.

“He’s talking about the enemy within our country, Pennsylvania,” Ms. Harris said to a jeering crowd. “He’s talking about that he considers anyone who doesn’t support him, or who will not bend to his will, an enemy of our country.”