Former Secretary of State Hillary Clinton shared concerns about ex-President Biden's political viability by 2024, a source said. (Getty Images)
Sullivan told Klain that Biden "was less effective in 2024 compared to 2022," the source said.
It's not immediately clear if Biden's mental acuity was the reasoning for their doubts, nor if they made the case to Klain together or separately.
But it's a significant indictment coming from top national Democrats of Biden in general, long before concerns about his fitness for office within the party were made public knowledge.
Sullivan had been a top aide to both Biden and Clinton, having served as the latter's senior policy advisor during her 2016 campaign.
Klain, who served as White House chief of staff for the first half of Biden's term, conceded that the then-president was less energetic and more forgetful, though he defended his "acuity to govern," the source said.
"Mr. Klain stated that President Biden often confused names and proper nouns, and it got worse over time," the source said.
Fox News Digital was told that Klain also said there was no reason to doubt President Donald Trump's own mental fitness.
House Oversight and Accountability Committee Chairman James Comer is leading the probe into Biden. (Anna Moneymaker/Getty Images)
Jeff Zients, who served as Biden's chief of staff for the final two years, was also asked to sit for a transcribed interview, a committee aide previously told Fox News Digital.
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A source familiar with the Biden team's thinking previously called Republicans' probe "dangerous" and "an attempt to smear and embarrass."
"And their hope is for just one tiny inconsistency between witnesses to appear so that Trump’s DOJ prosecute his political opponents and continue his campaign of revenge," that source said.
When reached for comment, Adrienne Watson, a representative for Sullivan, told Fox News Digital, "Jake did not have a conversation with Ron about Joe Biden running for president before the debate."
Fox News Digital also reached out to Klain's attorney as well as a contact for comment for Clinton but did not hear back by press time.
Elizabeth Elkind is a politics reporter for Fox News Digital leading coverage of the House of Representatives. Previous digital bylines seen at Daily Mail and CBS News.
Follow on Twitter at @liz_elkind and send tips to elizabeth.elkind@fox.com
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