Mr. Trump has had a string of unforced gaffes, garble and general disjointedness that go beyond his usual discursive nature, and that his Republican rivals are pointing to as signs of his declining performance.
On Sunday in Sioux City, Iowa, Mr. Trump wrongly thanked supporters of Sioux Falls, a South Dakota town about 75 miles away, correcting himself only after being pulled aside onstage and informed of the error.
It was strikingly similar to a fictional scene that Mr. Trump acted out earlier this month, pretending to be Mr. Biden mistaking Iowa for Idaho and needing an aide to straighten him out.
In recent weeks, Mr. Trump has also told supporters not to vote, and claimed to have defeated President Barack Obama in an election. He has praised the collective intellect of an Iranian-backed militant group that has long been an enemy of both Israel and the United States, and repeatedly mispronounced the name of the armed group that rules Gaza.
During a Sept. 15 speech in Washington, a moment after declaring Mr. Biden “cognitively impaired, in no condition to lead,” the former president warned that America was on the verge of World War II, which ended in 1945.
In the same speech, he boasted about presidential polls showing him leading Mr. Obama, who is not, in fact, running for an illegal third term in office. He erroneously referred to Mr. Obama again during an anecdote about winning the 2016 presidential race.
“We did it with Obama,” Mr. Trump said. “We won an election that everybody said couldn’t be won, we beat …” He paused for a beat as he seemed to realize his mistake. “Hillary Clinton.”
Last week, while speaking to supporters at a rally in New Hampshire, Mr. Trump praised Viktor Orban, the strongman prime minister of Hungary, but referred to him as “the leader of Turkey,” a country hundreds of miles away. He quickly corrected himself.
Why it matters: Trump, 77, often mocks the 80-year-old Biden as feeble and confused — even as some of Trump's foes are highlighting the former president's own gaffes and relatively light campaign schedule.
Florida Gov. Ron DeSantis, trying to cut into Trump's big polling lead in the GOP presidential race, has been targeting Trump's age.
- Last week, DeSantis' campaign revealed a "Trump accident tracker" to compile the former president's verbal slips on the trail, and asked whether Trump had the "stamina" to be president — using a word Trump often has invoked against his opponents, particularly Hillary Clinton in 2016.
- "This is a different Donald Trump than 2015 and '16 — lost the zip on his fastball," DeSantis told reporters in New Hampshire last week.
Trump's campaign has posted many videos of Biden stumbling, while Biden's campaign has answered with ads with Trump looking heavy and sweaty, often while golfing.