Donald Trump brags about "acing" a test only people with dementia fail.
Joe Biden rambles incoherently from his basement about nurses breathing in his nose.
Pollsters seriously ask who voters believe has the mental capacity to lead the nation.
With the US election now less than 100 days away, Americans must soon choose whether to give the oldest person to ever take the presidential oath of office a second term – or to break that record by a full eight years.
Trump, 73, was 70 on inauguration day in 2017.
If he wins in November, Biden, 77, will be 78 by the time he's sworn in.
In an attempt to calm younger voters, the former Vice President's campaign has quietly signalled that he would only serve one term and wouldn't run for re-election, according to Politico.
The Trump campaign is focusing heavily on the issue of Biden's mental fitness for the job, highlighting a number of instances in which the presumptive Democratic candidate appears virtually incoherent.
But some observers – and opinion polls – suggest that angle of attack is failing.
"I had nurses at Walter Reed hospital who would bend down and whisper in my ear, go home and get me pillows," Biden said during a virtual event this week.
"They would make sure they would actually – probably nothing ever taught, you can't do it in the Covid time – but they would actually breathe in my nostrils to make me move, to get me moving."
Donald Trump Jr highlighted the video on Twitter. "I'm no RN, but I can pretty definitively say this is not something a medical professional did," he said.
Podcast host Dave Rubin joked that the Biden campaign had a new slogan. "Biden 2020: Breathe in my nostrils. Get me moving."
Earlier this month, Biden seemed befuddled during the International Brotherhood of Electrical Workers' 2020 virtual conference.
"Lonnie knows I believe this every fibre of my being," he said.
"We're posed … I, what I proposed is, is … it can be done. I think we're in a position to, to really make it happen. And my team and your team already working closely together and liked, to light up the path forward here. Critical laws like the PRO Act to strengthen collective bargaining. Um, politics like prevailing, and, look, I guess I'm, I'm getting … I'm, I'm taking too much time, but, you know …"
In April, Fox News host Tucker Carlson highlighted another such instance from an interview Biden gave to ABC News about the Covid-19 crisis.
"We cannot let this, we've never allowed any crisis from the Civil War straight through to the pandemic of 17, all the way 'round, 16, we have never, never let our democracy sake, second fiddle, we can both have a democracy, elections, and at the same time correct the public health," he said.
Carlson quipped, "Ask yourself, is Joe Biden ready to lead this country?"
Late last year, another bizarre clip of Biden – this one from a June 2017 campaign event at a public pool in Wilmington, Delaware – went viral.
"And by the way, you know, I sit on the stand, and it get hot, I got a lotta, I got hairy legs, that turn … um, blonde in the sun," Biden said.
"And the kids used to come up and reach in the pool and rub my leg down so it was straight and then watch the hair come back up again, they'd look at it, so I learned about roaches, I learned about kids jumping on my lap, and I love kids jumping on my lap."
During an interview with Fox News host Chris Wallace last week, Trump was asked point blank whether Joe Biden was "senile".
"I don't want to say that – I'd say he's not competent to be president," Trump said.
"To be president, you have to be sharp and tough and so many other things. He doesn't even come out of his basement. Joe doesn't even know he's alive, okay? He doesn't even know he's alive."
In that interview, Trump bragged – not for the first time – about "acing" the "very hard" Montreal Cognitive Assessment, a test used as a screening tool to identify cognitive dysfunction, including early onset dementia and Alzheimer's disease.
Wallace appeared sceptical. "Well, it's not the hardest test," he said. "They have a picture and it says, 'What's that?' And it's an elephant."
Trump replied, "Yes, the first few questions are easy, but I'll bet you couldn't even answer the last five questions. I'll bet you couldn't. They get very hard, the last five questions."
Ziad Nasreddine, the Canadian neurologist who created the test in 1996, told The Washington Post the President's fixation on it was puzzling.
"It's not meant to measure IQ or intellectual skill in anyway," he this week.
"If someone performs well, what it means is they can be ruled out for cognitive impairment that comes with diseases like Alzheimer's, stroke or multiple sclerosis. That's it."
Nasreddine added, "The reason most people take the test is they or others start noticing mental decline. They forgot where they parked the car, can't remember what groceries to buy by the time they get to the store. They keep forgetting to take their medication."
In the Fox News interview, Trump insisted Biden should take the test.
"Biden can't put two sentences together," he told Wallace.
"They wheel him out, he goes up, he repeats, they ask him questions. He reads a teleprompter and then he goes back into his basement."
In a statement to The Washington Post, a Biden campaign spokesman dismissed the challenge. "The only testing Donald Trump should be focused on is the kind we need to get the Covid-19 crisis under control," he said.
Trump, for his part, has been mocked for repeated verbal gaffes and apparent difficulty pronouncing words – during his July 3 Mount Rushmore speech, he pronounced totalitarianism as "totaleetariotism" and Ulysses S. Grant as "you-licious".
Other highlights compiled by The Daily Show include "United Staysh", "stankchuary", "combat infantroopen", "transpants", "sta-ticks … su-tick-six", "slock rocket", "pivitible" and "heroilynn".
And according to a Fox News poll published last week, Trump's repeated attacks on Biden's mental fitness do not appear to be working.
The poll of 1104 registered voters found more people believed Biden has "the mental soundness to serve effectively as president" than Trump – 47 per cent to 43 per cent.
Even worse, 51 per cent of respondents said Trump was not mentally competent to be president, versus 39 per cent for Biden.
Writing in the Los Angeles Times, prominent anti-Trump conservative Jonah Goldberg argued Trump's strategy of attacking his rival as mentally incompetent was inherently risky.
"One of the central tasks of campaigning, and politics generally, is managing expectations," Goldberg wrote.
"As of now, all Biden has to do to beat the expectations laid out by Trump is prove he knows he's alive – a very light lift … (All) Biden will have to do is come across as a reassuringly normal, albeit gaffe prone, competent leader. Biden, despite his flaws, seems up to that."
He added, "If the Wallace interview is any indication, it's Trump who struggles to meet that remarkably low bar."