On Sunday, President Donald Trump said he was not joking about wanting a drug test for himself and Joe Biden before the first presidential debate this week.

“No. I’m not joking,” Trump said about the suggestion during a White House press conference. “I’m willing to take a drug test, I think he should too.”

The first presidential debate is set for Tuesday, September 29.

Trump On Biden’s Last Democratic Primary Debate Performance: ‘People Say He Was On Performance-Enhancing Drugs’

Trump said that in Democratic primary debates, Biden appeared to be “out of it” but then seemed to improve significantly before the former vice president’s final primary debate with Senator Bernie Sanders.

Referring to Biden, Trump said, “He was okay. I mean, he wasn’t great; he wasn’t terrible; he was okay.”

RELATED: Trump Suggests Biden Is Taking Drugs to Appear Coherent During Speeches

Trump seemed curious about how Biden could have improved so much despite his poor performances in earlier debates.

“People say he was on performance-enhancing drugs; a lot of people have said that, a lot of people have written that,” Trump said.

When journalists challenged the president on his sources for making such a claim about potential drug use by Biden, the President replied, “You can check out the internet, you’ll see, plenty of people say it.

“And whether he is or not, doesn’t matter; he can take a test and I can take a test too,” Trump insisted.

The Biden campaign responded to President Trump’s suggestion on Sunday.

Joe Biden “intends to deliver his debate answers in words,” Team Biden said. “If the president thinks his best case is made in urine he can have at it.”

Trump Reiterated His Call For A Drug Test Before In The Debate In A Sunday Tweet

Time and again, President Trump has wondered whether Biden has taken medication of some sort help him improve in debates.

Trump said last week, “Don’t underestimate him, look, he’s been doing this for 47 years.”

RELATED: The Only Cognitive Test Biden Needs to Take: Debate Trump

“You never know,” Trump added. “They give him a big fat shot in the a** and he comes out, and for two hours, he’s better than ever before.”

President Trump tweeted Sunday, “I will be strongly demanding a Drug Test of Sleepy Joe Biden prior to, or after, the Debate on Tuesday night. Naturally, I will agree to take one also.”

“His Debate performances have been record setting UNEVEN, to put it mildly,” he added. “Only drugs could have caused this discrepancy???”

How To Watch the First Presidential Debate

The first presidential debate of the 2020 election will be held on Tuesday, September 29 from 9-10:30pm Eastern.

You can view it on Fox News, or watch the livestream at NPR.

The debate is taking place at Case Western University in Cleveland, Ohio.