As former President Donald J. Trump continues to reach for attacks on his new opponent, Vice President Kamala Harris, that might halt her political momentum, he unveiled a new tactic at a rally in Bozeman, Mont., on Friday night, aiming to use Ms. Harris’s own words against her.

Interrupting his typical pattern of a digressive and lengthy speech, Mr. Trump played two video compilations of past remarks by Ms. Harris that his campaign hopes will portray her as overly liberal and inept.

The first video drew on statements that Ms. Harris made during the 2020 presidential campaign, when she tacked to the left and backed progressive ideas on criminal justice reform. The second was a montage of interviews and speeches that Mr. Trump’s campaign used to mock her speaking style and insult her intelligence.

The videos did little to alter the message that the Trump campaign has deployed against Ms. Harris for weeks and that Mr. Trump summed up during his speech on Friday.

“America cannot survive for four more years of this bumbling communist lunatic,” Mr. Trump told thousands gathered in the Brick Breeden Fieldhouse at Montana State University. “We cannot let her win this election.”

Mr. Trump and his allies have repeatedly tried to portray Ms. Harris as more liberal than President Biden in the three weeks since he ended his campaign and cleared the way for her to be the Democratic presidential nominee.