Eagle Pass, Texas - Donald Trump called Thursday for President Joe Biden to take a cognitive test, slamming the Democrat almost certain to be his opponent in November's presidential election for his "terrible decisions."

Trump regularly boasts about his performance on a simple exam meant to detect dementia, sparking ridicule from critics, but multiple missteps by both men have raised concerns over their advanced age and mental acuity.

"Crooked Joe Biden must take a Cognitive Test. Maybe that way we would be able to find out why he makes such terrible decisions," the 77-year-old posted on Truth Social as both he and the president made rival trips to the US-Mexico border.

"I took two of them, and ACED them both (no mistakes!). All Presidents, or people wanting to become President, should mandatorily take this test!"

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81-year-old Biden underwent an annual physical Wednesday and was declared "fit for duty" by doctors, who said there were no notable changes in his health over the past year.

The White House said Biden did not undergo a cognitive test because it was not deemed necessary by his doctors.

"He passes a cognitive test every day, as he moves from one topic to another topic, understanding the granular level of these topics," press secretary Karine Jean-Pierre said.

Polling has consistently shown that a majority of Americans do not want to see a rematch between Biden and Trump in November, deeming both too old for the White House.

Trump – who is expected over the coming month to secure the Republican nomination to run against Biden – often mocks Biden's verbal gaffes and impersonates his stiff gait during rallies.

The president was described in a recent report exonerating him over his retention of classified documents as "an elderly man" who frequently shows limitations with memory and recall.

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Trump boasts regularly about his performance in a dementia test that required him to remember five words in the correct order, but the Republican former president is also prone to regular verbal slip-ups.

In recent weeks, Trump has mistakenly referred to Hungarian Prime Minister Viktor Orban as the leader of Turkey and on one occasion repeatedly confused former US House speaker Nancy Pelosi with his primary rival, Nikki Haley.

"They both stink," he said after.