Entering the final stretch of the presidential race, Vice President Kamala Harris is focusing on a simple message that she believes will resonate with undecided swing state voters: that former President Trump mentally unfit for office.

Her argument is partly that the 78-year-old Trump has lost mental acuity on account of his advanced age, as was the chief line of attack that pushed President Biden from the race. But it is also that his perceived mental deterioration, which some say is evidenced by a string of bizarre incidents and rambling campaign speeches, would make Trump more dangerous than ever were he to win back the White House.

Trump has responded to the attacks in kind, saying it is Harris who is mentally unfit and repeating his claim she isn’t smart — a critique derided as sexist and racist by her supporters, but echoed by many of his. Trump campaign spokesman Steven Cheung said in a statement Friday that Trump has “more energy and more stamina than anyone in politics, and is the smartest leader this country has ever seen.”

In her own comments to reporters Wednesday, Harris said that Trump’s mental fitness was being questioned not only by her but by many Americans — including many of Trump’s former supporters, appointees and military leaders.

“Based on my observations, and I think the observations of many, Donald Trump is increasingly unstable,” Harris said. “And as has been said by the people who have worked closely with him, even when he was president, he’s unfit to be president of the United States.”

On Thursday, the Harris campaign launched a new ad running across battleground states that claims Trump would be “more unhinged, unstable and unchecked” than ever before if reelected, and would “ignore all checks that rein in a president’s power” in order to implement Project 2025, a blueprint for a new Trump term drafted by many of his former aides and allies.

The ad is based in part on internal campaign data showing that the argument that Harris is a stable alternative to an erratic Trump is an effective one — including among the undecided swing state voters her campaign is trying to reach.

Public polling has shown Trump is losing ground in terms of how voters view his mental fitness, with the percentage of Americans who believe he is mentally sharp declining, and the percentage who think he is too old for the job increasing.

The heightened focus on Trump’s mental acuity flips the script from when Biden was in the race. Biden, 81 and the nation’s oldest sitting president, ceded the Democratic ticket to Harris, who turns 60 on Sunday, after concerns about his age and acuity mounted following a disastrous debate against Trump in June.

Now it is Trump who would hold the distinction of being the oldest sitting president if he wins and serves out the full term.

Trump has mixed up words, names, places and timelines in his remarks on the campaign trail and in interviews, and routinely goes on strange tangents in the midst of longer and longer stump speeches.

On Monday, a Trump town hall event took a particularly bizarre turn after some members of the crowd had medical emergencies, as the candidate decided to cut off questions and simply dance to music on stage for more than half an hour. Video of him swaying awkwardly in front of a restless crowd went viral.

The Harris campaign tweeted a video compilation from the event, writing, “Trump appears lost, confused, and frozen on stage as multiple songs play for 30+ minutes and the crowd pours out of the venue early.” Harris’ own post on X — in which she simply wrote, “Hope he’s okay” — has since been reposted more than 250,000 times.

Last week, more than 230 doctors, nurses and health care professionals, many of whom back Harris over Trump, issued a public letter calling on Trump to release his medical records — as Harris has done and Trump has promised but failed to do.

The medical professionals wrote that without such records, they were left to judge Trump’s mental acuity based solely on his public appearances — and that “on that front, Trump is falling concerningly short of any standard of fitness for office and displaying alarming characteristics of declining acuity.”

They noted, among other things, Trump’s tendency to “ramble, meander, and crudely lash out at his many perceived grievances.”

Concerns from the medical community about Trump’s mental fitness are not new. Last month, a coalition of mental health experts and doctors convened at the National Press Club in Washington for a conference on Trump and the unique threat they believe he poses to the country and the world.

The same group published a bestselling book in 2017 titled “The Dangerous Case of Donald Trump: 27 Psychiatrists and Mental Health Professionals Assess a President.” Now they were back with a new book, titled “The More Dangerous Case of Donald Trump: 40 Psychiatrists and Mental Health Experts Warn Anew.”

Dr. Bandy Lee, a forensic and social psychiatrist and editor of the project, said Trump shows concerning signs of potential mental illness and deterioration, and should agree to an independent psychological evaluation, just as military commanders are required to do.

Lee said it is concerning that Trump is canceling interviews that aren’t with friendly outlets and avoiding settings where his mental deterioration might be evident, such as another debate against Harris.

“We’re seeing a lot of signs — odd behavior — and people doing all kinds of things to cover for his impairments,” Lee said.

Trump has canceled some engagements, but is not hiding from the public.

On Thursday evening, Trump spoke at an annual Catholic charity dinner in New York, where he joked about facing criminal subpoenas in the state and described Harris’ decision to skip the event, which traditionally features light roasts between the candidates, as “very disrespectful.”

Trump also mocked Harris’ laugh, mispronounced her name multiple times, and said he will like her more once people “dispose of her.”

In response, the Harris campaign stayed on message. Rapid-response director Ammar Moussa, in a statement, said Trump “struggled to read scripted notes written by his handlers,” “stumbled over his words and lashed out when the crowd wouldn’t laugh with him,” and “went on long, incomprehensible rambles” when off script — “reminding Americans how unstable he’s become.”

Trump has released poorly detailed but glowing evaluations of his health by Rep. Ronny Jackson, Trump’s former White House physician and now a congressman from Texas, but they have been met with deep skepticism and calls for a more independent analysis.

Cheung, Trump’s spokesman, said in his statement to The Times that Trump “does multiple public events every single day and the public can see he is sharper and more focused than ever before because the future of America is at stake.” Voters, he said, can compare that to “the stupidity and incompetence of Kamala Harris that is on display and is an embarrassment to the rest of the world.”

At Thursday’s event, Trump made a joke that also turned the criticisms back on Harris. “Right now we have someone in the White House who can barely talk, barely put together two coherent sentences, who seems to have the mental faculties of a child. It’s sad. This is a person who has nothing going, no intelligence whatsoever,” he said. “But enough about Kamala Harris.”

Trump has also provided an alternative explanation for his rhetorical tangents — which he calls “the weave” and a sign of “genius.”

“You know, I do a thing called ‘the weave,’” Trump said on the “Flagrant” podcast this month. “And there are those that are fair that say, ‘This guy is so genius.’ And then others would say, ‘Oh, he rambled.’ I don’t ramble.”

He claims he has an “extraordinary memory” that allows him to pivot to different topics in a single conversation or speech before returning to his original point.

“I can go so far here or there, and I can come back to exactly where I started,” Trump said, causing the podcast’s comedian co-hosts, Andrew Schulz and Akaash Singh, to burst into laughter.