"I am all in. I'm riding with Biden no matter which direction he goes," Clyburn said on NBC's "Today" program.
Clyburn, 83, is a respected voice among Black Americans whose support is essential to Biden's 2024 campaign. He has served in Congress for more than 30 years and played a leading role in Biden's successful 2020 White House run.
However, another congressional Democrat called on Biden to step aside and allow the party to pick another standard bearer, raising to 18 the number who have done so.
"Please pass the torch to one of our many capable Democratic leaders so we have the best chance to defeat Donald Trump," Representative Brittany Pettersen wrote on social media.
Democratic officeholders, donors and activists are trying to determine whether Biden is their best bet to defeat Republican Donald Trump in the Nov. 5 election and serve another four-year term in the White House.
With most U.S. voters firmly divided into ideological camps, opinion polls show the race remains close.
Hakeem Jeffries, the Democratic leader in the House of Representatives, said he met with Biden on Thursday night to convey the range of thoughts they held about his candidacy. He did not say how Biden responded.
"I directly expressed the full breadth of insight, heartfelt perspectives and conclusions about the path forward," Jeffries wrote in a letter to colleagues.
Democrats are worried that Biden's low approval ratings and growing concerns that he is too old for the job could cause them to lose seats in the House and Senate, leaving them with no grip on power in Washington should Trump win the White House.
Thursday's press conference provided fodder for Biden supporters and doubters alike.
At one point, Biden referred to his vice president, Kamala Harris, as "Vice President Trump". Hours earlier he introduced Ukrainian President Volodymyr Zelenskiy as "President Putin" at the NATO summit, drawing gasps from those in the room.
Biden occasionally garbled his responses at the press conference but also delivered detailed assessments of global issues, including Ukraine's war with Russia and the Israel-Gaza conflict, that served as a reminder of his decades of experience on the world stage.
A senior Biden campaign official who spoke on condition of anonymity called the performance the "worst of all worlds. Not good. But not bad enough to make him change his mind."
Another strategist who worked on Biden's 2020 campaign said it would not quell Democrats' concerns.
Fundraiser Dmitri Mehlhorn said other donors told him they saw a strong performance from the president. "This is the person who can beat Trump. The mistakes are baked in and the upside is strong," he told Reuters.
Biden will try to shift the focus back to Trump at an evening rally in Detroit.
He and other Democrats have warned that a sweeping policy agenda crafted by conservative allies called Project 2025 would give Trump a blank check to pursue his whims. Trump has distanced himself from the project.
After two weeks of fallout from Biden's halting debate performance, Democrats are hoping the bright spotlight shifts to Trump and his agenda next week when the Republican Party convenes in Milwaukee to formally nominate him for president.
The Michigan city is also headquarters of the United Auto Workers labor union, whose leaders endorsed Biden but now are assessing their options, three sources told Reuters.
An NPR/PBS poll released on Friday found Biden leading Trump 50% to 48%, a slight increase from his position before the debate. Biden fared slightly worse than Trump when third-party candidates were included in the questioning.
