US President Donald Trump has called for himself and Democratic challenger Joe Biden to submit to drug tests before their first debate next month.

Mr Trump told the Washington Examiner he had noticed a sudden improvement in Mr Biden’s performance in the Democratic TV debates.

“Nobody thought that he was even going to win,” Mr Trump said.

“Because his debate performances were so bad.”

The US President’s call comes as Mr Biden confirmed he will not skip debates, vowing he will use the opportunity to confront his rival and be a “fact-checker on the floor”.

Mr Biden says there is no question the debates will take place after House Speaker Nancy Pelosi told reporters earlier on Thursday (local time) she did not think Mr Biden should debate the President at all.

Ms Pelosi said she did not think he should “legitimise a conversation” with Mr Trump because he would “probably act in a way that is beneath the dignity of the Presidency”.

Donald Trump has expressed suspicion at Joe Biden’s debate performance. Photo: AP

The Democratic nominee has repeatedly said he is eager to take on the President.

“Here’s the deal with bullies, I understand how they work,” Mr Biden told CNN.

“And I’m going to play by the rules of the debate commission and we’re going to have a debate.”

In an interview on MSNBC, Mr Biden said he was going to be a “fact-checker on the floor” during the debates and he thought the media would fact-check Mr Trump as well.

Ms Pelosi said a 2016 debate between Mr Trump and Democrat Hillary Clinton was “disgraceful” as he stood close behind Ms Clinton as she spoke and should have been told to move away.

Instead, Ms Pelosi suggests the two candidates have individual events where they take questions.

Mr Biden and Mr Trump will have three debates before the November 3 election.

The first debate is on September 29.

-with agencies