US President Joe Biden returns to the campaign trail on Friday, pushing ahead with his reelection bid after a mixed performance at a major news conference failed to silence calls for him to quit.

The 81-year-old will give a speech bashing rival Donald Trump in Detroit in the rust-belt state of Michigan, a crucial battleground state that the Democrat must win in November's election.

A defiant Biden insisted in a pivotal appearance at a Nato summit on Thursday that he would run again, and win – despite concerns about his age and health following a disastrous debate performance two weeks ago.

But a series of gaffes, including referring to Vice President Kamala Harris as "Vice President Trump," kept Biden's fitness for a second term under the microscope.

The top Democrat in the House of Representatives, Hakeem Jeffries, said he had met with Biden late on Thursday, as the number of members calling on the president to step aside rose to 20.

Jeffries said he and Biden "expressed the full breadth of insight, heartfelt perspectives and conclusions about the path forward" but did not give further details.

The president has faced a steady drumbeat of Democrats calling for him to abandon his 2024 candidacy since the June 27 debate, during which Biden often lost his train of thought and appeared tired.

But Biden has obstinately dug in and insists that he can convince voters to back him despite most polls showing him trailing the criminally convicted, twice-impeached Trump.

The campaign event in Detroit is his fourth trip this year to the state, part of the industrial "blue wall" along with Wisconsin and Pennsylvania that were key to his 2020 win against his rival.

Biden's speech is expected to focus on "Project 2025", a blueprint by hardline conservatives for the first days of a Trump term that Democrats have pinned on the former president, despite the 78-year-old's denials.

The press conference on Thursday appeared to buy Biden some time even if it was far from a knockout blow, with three more lawmakers urging him to quit just minutes after it finished.

Biden said he was the "most qualified person to run for president," rejecting calls for him to step aside before the November 5 vote.

He acknowledged he had to "allay fears" in the Democratic party and should "pace myself a little more" after blaming jet lag and a cold for his stumbling debate performance.

Biden also fielded a series of foreign and domestic policy questions with detailed if meandering answers and relatively few slip-ups, though he did mix up Europe and Asia.

But there were damaging moments on Thursday, with the Trump-Harris mixup and an earlier gaffe at the Nato summit in which he introduced Ukraine's President Volodymyr Zelensky as Russian strongman Vladimir Putin.

The concerns over Biden are also affecting Democratic donors, with Hollywood star and high-profile supporter George Clooney calling on Biden to step aside on Wednesday.

A number of other key donors have told the largest Biden campaign fund that around US$90 million in pledged donations is on hold if he carries on running, the New York Times reported on Friday. (AFP)