Mary Trump believes her uncle, President Donald Trump is growing desperate to improve his reelection chances.

That's why he accused Joe Biden of taking drugs to enhance his performance before debates, the author of "Too Much and Never Enough" told CNN Chief Media Correspondent Brian Stelter on "Reliable Sources" Sunday.

The drug-use accusation is "Donald's desperation," Mary Trump said, adding that "he's grasping at straws at the moment."

President Trump has accused Biden of drug use in the past, although there is no evidence to substantiate the claim. The accusation began picking up buzz on social media Friday, after Trump spoke to Fox News host Jeanine Pirro.

"There's probably, possibly drugs involved. That's what I hear," President Trump told Pirro.

Mary Trump said her uncle has accused Biden of mental instability because the president himself has been accused of lacking mental fitness in his old age.

Trump has defended himself by claiming he aced a cognitive test, although that does not -- as he claimed -- indicate mental superiority.

"He projects about almost everything," Mary Trump said.

"Normalizing him — which has been going on for decade — is one of the reasons we're here"

Mary Trump said she has no information about her uncle using drugs.

As for Trump's approach to the coronavirus, which is being scrutinized after excerpts from Bob Woodward's book "Rage" were released, Mary Trump said enough evidence exists to condemn the president's response to the pandemic without needing to know what was said behind closed doors.

"What we need to focus on is what Donald actually said and we can deal with the Woodward issue later," she said.

She said she wants her uncle to be held accountable for his actions. But she noted in the past that the president is haunted by his upbringing.

"Donald is a psychologically deeply damaged man, based on his upbringing and the situation with his parents," she told CNN's Chris Cuomo  in July.