Early in his speech in Minnesota on Saturday night, former President Donald J. Trump made clear just how quickly he has jettisoned the appeal for national unity that he made after he survived an assassination attempt in Pennsylvania two weeks ago.

“I want to be nice,” Mr. Trump said. “They all say, ‘I think he’s changed. I think he’s changed since two weeks ago. Something affected him.’”

But to a cheering crowd of thousands, Mr. Trump quickly conceded the point. “No, I haven’t changed,” he said. “Maybe I’ve gotten worse. Because I get angry at the incompetence that I witness every single day.”

Propelled by the upheaval in the presidential race caused by President Biden’s decision to end his campaign six days ago, Mr. Trump on Saturday once more escalated his attacks against Vice President Kamala Harris, now the presumptive Democratic nominee.

During a speech lasting roughly 90 minutes, Mr. Trump called Ms. Harris “evil,” “unhinged” and “sick.” He lied about her views on abortion in an effort to paint her as extreme, and he mocked her laugh and her demeanor.

“We have a brand-new victim,” Mr. Trump told thousands of people inside the Herb Brooks National Hockey Center in St. Cloud, Minn. “And, honestly, she’s a radical left lunatic.”

Mr. Trump spent considerable time attacking Ms. Harris’s views on public safety, taking aim at her efforts to portray herself as a “rule of law” prosecutor who contrasts starkly with Mr. Trump’s two impeachments, four criminal indictments and 34 felony convictions.

As he rallied some 60 miles from Minneapolis, where the killing of George Floyd in 2020 prompted a movement for criminal justice reform, Mr. Trump accused Ms. Harris of backing soft-on-crime policies, including a push to defund the police.

Ms. Harris told The New York Times in 2020 that she supported the “defund the police” movement’s idea of rethinking “what public safety looks like” and the size of police budgets. “But, no, we’re not going to get rid of the police,” she said. “We all have to be practical.”

But Mr. Trump, who throughout his third campaign for president has cloaked himself in support for law enforcement even as he grapples with criminal cases, used Ms. Harris’s past support of criminal justice reform to insist that he was “going to over-fund” the police.

Mr. Trump’s focus on public safety and his accusations that Democrats have allowed crime to run rampant in cities have been at the heart of his three political campaigns. His return to that message in Minnesota demonstrated how central his plea to law and order will most likely be to his effort to win over moderate and independent voters.

Mr. Trump and his team are eager to flip Minnesota, which last voted for a Republican president in 1972, but which also has a large population of working-class voters and union workers, groups that Mr. Trump drew support from in his previous elections. He lost the state by just 1.5 percentage points in 2016, only to lose it by a wider margin four years later.

Even as the race has changed dramatically, in St. Cloud, Mr. Trump drew on the same themes that have been animating his campaign all year: protectionist trade policies, an enormous crackdown on immigration and his relentless repetition of his false claims of widespread voter fraud in the 2020 election.

“Anybody that can cheat on elections like they cheat on elections, these are not stupid people,” Mr. Trump said of Democrats, even though there is no evidence to support his claims.

Still, Mr. Trump’s speech highlighted his struggle to adapt to a new opponent after years of preparing to face Mr. Biden. Though Mr. Biden is no longer on the Democratic ticket, Mr. Trump revived his derisive impressions of the president, caricaturing his gait and speech to suggest that Mr. Biden is not fit for office.

At one point in his speech, Mr. Trump appeared about to imitate Mr. Biden but then stopped himself. “I don’t want to waste a lot of time on it,” he said, “because it’s over now, right? He’s gone.” But he gloated: “I told you that he would be. I told you that he wasn’t going to make it.”

Sarafina Chitika, a spokeswoman for the Harris campaign, criticized Mr. Trump’s focus on Mr. Biden. “Tonight in Minnesota, a bitter, unhinged, 78-year-old convicted felon kept clinging to his lies about the 2020 election he lost being ‘rigged,’ rambled about his former opponent and golfing, and made excuses for why he’s afraid to debate Vice President Harris,” she said in a statement.

Saturday’s rally was Mr. Trump’s second joint rally with Senator JD Vance of Ohio since he chose Mr. Vance to be his running mate. Mr. Vance largely echoed Mr. Trump’s attacks, calling Ms. Harris overly liberal and a “card-carrying member of the San Francisco lunatic fringe.”

And Mr. Vance, whose rollout as the Republican vice-presidential nominee has not been wholly smooth, attacked the press for not being sufficiently critical of Ms. Harris.

“The media told us that Joe Biden was Abraham Lincoln,” Mr. Vance said. “And now the media tells us that Kamala Harris is Martin Luther King Jr.”