President Trump has struggled to pronounce the United States' most famous National Park, labouring over the word Yosemite in a speech at the White House.

Trump was signing the Great American Outdoors Act, legislation that will devote nearly US$3 billion annually to conservation, when he mispronounced the name of the park.

Speaking about the need to preserve the American wilderness, Trump started strongly enough by telling the crowd: "We want every American child to have access to pristine outdoor spaces, where young Americans experience the breathtaking beauty of the Grand Canyon."

When he turned his attention to Yosemite National Park, the wheels fell off.

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"When their eyes widen in amazement as Old Faithful burst into the sky, when they gaze upon Yo-semites, Yo-seminites, towering sequoias, their love of country grows stronger and they know that every American has truly a duty to preserve this wondrous inheritance," Trump said.

Trump was widely mocked online for the gaffe, coming as public attention has been focused on the mental acuities of both presidential candidates.

Late last month, 74-year-old Trump attempted to demonstrate his mental fitness by reciting five words — in order, importantly — over and over in a television interview.

The President said that collection of nouns, or ones like them, was part of a cognitive test he had aced while declaring that his likely Democratic opponent, 77-year-old Joe Biden, could not do the same.

In a battle of septuagenarians, the Trump campaign has long tried to paint Biden as having lost some of his mental sharpness. But the gambit has yet to prove successful in denting the former vice president's standing in the race.

That leaves Trump trying to escalate the attacks while defending his own ability to handle the mental rigours of the job.

"The first questions are very easy," Trump told Fox News. "The last questions are much more difficult. Like a memory question. It's, like, you'll go: Person. Woman. Man. Camera. TV. So they say, 'Could you repeat that?' So I said, 'Yeah. It's: Person. Woman. Man. Camera. TV'."

He then recalled that, at the end of the test, the doctor asked him to recite it again.

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"And you go: 'Person. Woman. Man. Camera. TV.' If you get it in order, you get extra points," Trump said. "They said nobody gets it in order. It's actually not that easy, but for me, it was easy."

Trump boasted that he dazzled the doctors because he has "a good memory, because I'm cognitively there" and delivered an unsubtle accusation about Biden.

"Now Joe should take that test because something's going on," Trump said. "And, I say this with respect. I mean — going to probably happen to all of us, right? You know? It's going to happen."

-Additional reporting, AP