Washington DC - Congressman Lloyd Doggett became the first Democratic lawmaker to publicly call on Joe Biden to make way for another candidate on Tuesday after the president's dismal debate performance.

Biden's own side has finally started expressing their growing doubts about the 81-year-old following last week's televised showdown with Donald Trump in which Biden stumbled over his words and lost his train of thought – and exacerbating fears about his age.

"Recognizing that, unlike Trump, President Biden's first commitment has always been to our country, not himself, I am hopeful that he will make the painful and difficult decision to withdraw. I respectfully call on him to do so," the Texan said in a statement.

Nancy Pelosi, former House speaker and a grandee of the Democratic Party, said in her own statement it was "legitimate" to ask whether Biden's debate disaster was indicative of a deeper problem rather than a one-off.

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She praised Biden's vision and "strategic thinking" in an interview with cable network MSNBC but admitted he had had a "bad night" and said it was fair to raise the mental acuity of "both candidates."

Democratic Senator Sheldon Whitehouse of Rhode Island also asked for reassurances, saying voters needed to know there would be no repeat of Biden's sub-par showing.

Whitehouse told WPRI-TV he was "pretty horrified" by the president's performance during the 90-minute CNN match-up, watched by more than 50 million Americans.

Biden has not given a live interview or held a press conference since the debacle, meaning he has not had to give unscripted comments under pressure again.

On Monday, at the end of a brief speech about a Supreme Court decision, a journalist asked the president if he planned to withdraw from the race for the White House.

Biden left the room without answering.

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The polling margins between the president and his Republican predecessor have been razor-thin and almost static for months, with Trump showing a slight advantage in the all-important swing states.

Biden pushed for an unusually early first debate in hopes that he could jolt the race while there was still time to build on any momentum gained – but the plan backfired.