Bernie Sanders' campaign is accusing Joe Biden of pushing for a sit-down format at the March 15 presidential primary debate in Phoenix, according to a new report that highlights mounting concerns over the 77-year-old former vice president's stamina and cognitive ability.

Sanders' team raised the alarm about the sit-down format during a Friday call with CNN, which is set to moderate the debate along with the Democratic National Committee (DNC), Politico reported this weekend. All previous primary debates this cycle have required candidates to stand at podiums; and during his one-on-one debates with Hillary Clinton in 2016, Sanders stood alongside her.

"Why does Joe Biden not want to stand toe-to-toe with Sen. Sanders on the debate stage March 15 and have an opportunity to defend his record and articulate his vision for the future?" senior Sanders advisor Jeff Weaver told the outlet.

The Biden campaign denied it had anything to do with the proposed new format. Biden has stood for each of the previous debates, but the upcoming one-on-one with Sanders is expected to be a more grueling and taxing event. (Rep. Tulsi Gabbard, D-Hawaii, will be excluded from the debate under new DNC rules requiring that each candidate have received a large number of delegates.)

"We will participate in whatever debate CNN chooses to stage: standing, sitting, at podiums or in a town hall," Biden deputy campaign manager Kate Bedingfield told Politico. "The problem for the Sanders campaign is not the staging of the debate, but rather, the weakness of Sen. Sanders’ record and ideas."

Bedingfield added that during the joint call with CNN and the campaigns, the Biden team "agreed to the format that CNN proposed" -- and that after Sanders objected, the Biden "campaign agreed to a modification of that format that CNN proposed, to compromise with both campaigns."

The nature of that modification was unclear, but the DNC has suggested the format would change to become more intimate. Sit-down-style debates are not unprecedented; in 2012, Biden sat across from then-Rep. Paul Ryan at a vice presidential debate, and in 2008, the-candidate Barack Obama debated Clinton at a roundtable.

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"After 10 debates, the DNC worked with its network partners to adapt the March debate to the smaller field of candidates and to give voters more of a voice. This format provides candidates longer response times, and for the first time, will incorporate questions from undecided voters in the audience," DNC spokeswoman Xochitl Hinojosa said.

Thus far, Biden's debate performances have been largely underwhelming, analysts have said. He has suggested that most Americans owned "record players" and could use them to connect with children, declared that half of the country has been killed by gun violence since 2007, and baffled audience members by telling them to "go to Joe 30330."

Republicans, meanwhile, have made Biden's possible health issues a major part of their messaging, and there were signs that the attacks weren't simply political. Obama's longtime doctor, for example, has flatly remarked in an interview with The Washington Examiner that Biden was "not a healthy guy."

"He's not in bad shape for his age, but I wouldn't say he’s in outstanding health," Dr. David Scheiner said last year, after reviewing partial medical records. "Could I guarantee he won't have issues for the next four years? He has a lot of issues that are just sort of sitting there."

On Sunday, Trump mocked Biden's gaffe-marked, seven-minute speech in St. Louis the day before.

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During the brief address, Biden declared: "If you want a nominee who will bring this party together, who will run a progressive, positive campaign, and turn, turn this primary from a campaign that's about negative attacks into one that's about what we're for -- because we cannot get -- re-elect -- we cannot win this re-election -- excuse me. We can only re-elect Donald Trump -- if, in fact, we get engaged in this circular firing squad here. Gotta be a positive campaign, so join us."

Biden also bizarrely described himself as an "O'Biden Bama" Democrat, transposing his name and the name of his old boss.

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And, Trump suggested at a Fox News Town Hall last week that Biden wasn't fit for office, saying that Biden's numerous gaffes were indicative of a competency issue.

"I'm all set for Bernie, communist," Trump began. "And then, we have this crazy thing that happened on Tuesday, which he thought was Thursday. But, he also said 150 million people were killed with guns, and that he was running for the U.S. Senate -- there's something going on there."