Joe Biden is racking up endorsements from former rivals and Democratic insiders, but a gaffe-riddled 7-minute speech on Saturday in St. Louis, Missouri, led President Trump to tweet out an edited clip that appears to have the former vice president endorse the current commander in chief.

During the brief address, Biden, 77, 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.

The muddled remarks led to renewed concerns from both Republicans and left-of-center commentators that Biden may be mentally unfit for the presidency. On Twitter, President Trump jokingly took Biden's speech as a mangled endorsement.

"I agree with Joe!" Trump wrote, captioning a video of the address that edited out the second part of Biden's remarks.

The tweet was part of a larger effort by Republicans to point out what they see as one of Biden's fatal vulnerabilities, along with his son's possible corruption. Trump suggested at a Fox News Town Hall last week that Biden isn'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."

With Biden set to potentially deliver a knockout blow to Bernie Sanders during Tuesday's Michigan primary, some doctors and even some left-of-center commentators are agreeing with the president's assessment.

Former President Obama's longtime physician has flatly remarked in an interview with The Washington Examiner that Biden is "not a healthy guy."

"He’s not a healthy guy." — Dr. David Scheiner, longtime Obama physician, on Joe Biden

“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.”

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Popular podcast host Joe Rogan, who has endorsed Bernie Sanders, took the concerns a step further, saying Biden's alleged cognitive problems were disqualifying.

"Stop. Pause. He can't be president," Rogan said. "Listen, we can't play any games here folks. This is a really old man who can't talk."

On Sunday, Glenn Greenwald countered accusations on social media that he was mentioning what he called Biden's "obvious cognitive issues" only to help Sanders, noting that he has been writing about them for months.

Journalist Aaron Mate has pointed out the relative lack of scrutiny for Biden in the network media, which with some exceptions has largely steered clear of probing the former vice president's overseas dealings or his possible cognitive issues.

"When's the last time Joe Biden sat for a live, unedited interview like Bernie did this week w/ Maddow?" Mate asked. "For example, I'd love to know what Biden said here after claiming Hunter got his $80K/month Ukraine board gig because 'he's a really bright guy.' But it's clearly edited."

And, last summer, The New York Times reported that many Biden associates and insiders were doubting with "significant unease" whether he would be too old to serve as an effective candidate.

Meanwhile, reports have emerged that the format for the next presidential debate will allow Biden and his opponent to sit down -- prompting the Sanders campaign to suggest Biden is getting favorable treatment.

“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?” Jeff Weaver, Sanders’ senior adviser, told Politico.

For now, though, Biden is largely basking in the support of a mostly unified Democratic Party -- even picking up the endorsement of Sen. Kamala Harris, D-Calif., who just last year accused Biden of supporting what she called racist policies that affected her personally.

"There is no one better prepared than Joe to steer our nation through these turbulent times, and restore truth, honor, and decency to the Oval Office," Harris said in a statement Sunday. "He is kind and endlessly caring, and he truly listens to the American people."

Harris joins Sen. Amy Klobuchar, D-Minn., former South Bend, Ind., Mayor Pete Buttigieg, former Texas Congressman Beto O'Rourke, ex-New York City Mayor Mike Bloomberg, Rep. Tim Ryan, D-Ohion, former Massachusetts Gov. Deval Patrick, and former Maryland Congressman John Delaney in endorsing Biden.

Harris withdrew from the race in December, ending a candidacy with the historic potential of becoming the first black woman elected president.

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Harris said in her statement that "like many women, I watched with sadness as women exited the race one by one." Four years after Hillary Clinton was the party's nominee, "we find ourselves without any woman on a path to be the Democratic nominee for president."

Rep. Tulsi Gabbard, D-Hawaii, remains in the race, but with only a single delegate so far, has been viewed as a long-shot at best.

Fox News' Ronn Blitzer and The Associated Press contributed to this report.