Donald Trump has long attacked Joe Biden, his likely opponent at the polls next year, as "Sleepy Joe", portraying the 80-year-old president as too old and too mentally fogged to occupy the Oval Office. As recently as Friday, the former president attacked his successor for being unfit to deal with Russia and the threat of nuclear war.
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But Trump's tactics rebounded when he said Biden threatened to lead the US into "world war two" - and suggested that he, Trump, thought he had beaten Barack Obama for the presidency back in 2016.
There have been two world wars. The first ended in 1918, the second in 1945. The cold war, the nuclear standoff between the US and the Soviet Union that often threatened a third world war, ended with the fall of the communist regime in Moscow in 1991.
Obama was president, and Biden vice-president, from 2009 to 2017. In the 2016 election, Trump beat Hillary Clinton.
Mockery of Trump's stumbles was immediate and sustained. But it also pointed to an increasingly stark issue on both sides of the aisle: the advanced age of many American leaders, and polling that shows most voters want generational change.
At 80, Biden is the oldest president ever. Should he win re-election and serve a full term, he will be 86 on leaving office. Polling has shown more than 75% of Americans think he is too old for a second term.
Trump is 77 but polls show significantly fewer voters think he is too old to return to power. Whether gaffes like those he made in Washington move the needle remains, of course, to be seen.
Addressing the Pray, Vote, Stand summit, a rightwing event, Trump said Biden was "cognitively impaired, in no condition to lead and ... now in charge of dealing with Russia and possible nuclear war".
Under Biden, he added: "We would be in world war two."
On Monday, the MSNBC host Joe Scarborough, a former Republican congressman, laughed as he said: “It's almost like it's the summer of 1939 all over again. You know, [Trump's] father's going to a Nazi rally or something, or a Klan rally. I don’t know which rally he did or didn’t go to.”
Trump's father, Fred Trump, was arrested after a Klan riot in Queens, New York, in 1927. Donald Trump has reportedly expressed sympathy for Nazism and Adolf Hitler.
"But yeah," Scarborough said. "You think they may want to take out the 'cognitively impaired' part of his speeches from now on."
Jonathan Lemire, his fellow host, said: “That's an attack line the Republicans and Trump love to use [against Biden] but, man, that does seem like he was looking in the mirror right there.