Palm Beach, Florida - Donald Trump challenged President Joe Biden to take a drug test ahead of their much-anticipated first presidential debate on Thursday.

Saying he would also agree to undergo the test, Trump shared a letter written by Texas Representative Ronny Jackson on his Truth Social platform.

The Republican and former presidential physician's letter accused Biden of taking "performance enhancing drugs" ahead of "high stakes events like the State of the Union," and demanded the 81-year-old take "a clinically validated" test before the candidates take the stage for the CNN-hosted event.

The New York Post cited a source close to Trump who said: "There is no universe in which [Biden is] not being medicated before the debate. If he wants to dispel that, he should offer to take a drug test, but we all know he won't do that."

Trump and his allies have regularly attacked Biden over his age and behaviors they say are indicative of mental decline.

On the other hand, the 78-year-old former president has also faced questions over his fitness for office. Trump's speeches often devolve into rambling anecdotes, and he has more than once gotten prominent figures confused. Most recently, he got Ronny Jackson's name wrong while boasting about the results of a cognitive assessment.

"The other day, you may remember, he was trying to question our president’s mental acuity, and he could not remember the name of his own doctor, so tell President Trump bring whatever he’s got," Biden campaign co-chair Mitch Landrieu told NBC Sunday. "President Biden will be standing there ready for him."

The first presidential debate will take place in Atlanta, Georgia, airing at 9:00 PM ET.