As Joe Biden prepares to take the mantle of presumptive Democratic nominee, he's facing a new round of questions about his mental acuity at age 77.

The questions have come from both the former vice president's left and right. Since his decisive wins on Super Tuesday, President Trump has suggested Biden isn't all there mentally. Over Twitter and in brief interviews with reporters, Trump has jabbed at Biden, saying he's "lost his fastball," and is not "playing with a full deck." Last week, Trump said Biden would "destroy" entitlement programs without "even know[ing] he’s doing it."



Trump focused extensively on Joe Biden during his comments at various fundraisers this weekend at his Florida club. Talked about Biden as likely nominee & attacked his mental acuity extensively, saying Biden often had no idea what state he was in, per several attendees. — Josh Dawsey (@jdawsey1) March 9, 2020



But supporters of socialist Vermont Sen. Bernie Sanders, Biden's remaining rival in the Democratic race, have also raised questions about the 36-year Delaware senator.

"I don’t think we even need to talk about it; I think people can see it. By the way, people are seeing it and talking about it, and the response by Biden's supporters is: 'Let’s not have a discussion what everyone is saying and seeing?'" said Sanders supporter and liberal activist Nomiki Konst. "We have to say to ourselves, 'Who is the strongest candidate to take on Trump? Who can take on Trump’s debate behaviors?' Trump’s willing to throw anything at a candidate."

For decades, speculating about a candidate's mental health was a no-no, as it violated the "Goldwater Rule," dating to the 1964 presidential race. Fact magazine that year published an article about the Republican nominee, an Arizona senator, "The Unconscious of a Conservative: A Special Issue on the Mind of Barry Goldwater." The outlet polled psychiatrists about Goldwater's mental health and whether he was fit to be president. Prohibition on that practice was added to the American Psychiatric Association's Principles of Medical Ethics in 1973.

Biden's allies scoff at any claim that he's not functioning at 100%, and many say that any suggestion otherwise is part of a Russian disinformation campaign. His tendency toward verbal gaffes is nothing new, they note.

[ Related: Giuliani: Biden showing 'obvious signs of dementia']

And not all Sanders supporters are thrilled at the idea of Biden's fitness coming under attack.

"The fact that he’s been able to keep up with this campaign tells you he has the stamina. He’s just not Obama in terms of his ability to speak as eloquently. There’s an ecosystem out there of how we get the news, whether it’s Twitter or Facebook and that is where we need to be discerning and recognizing personal attacks versus ones based on policy," said Megan Ottens-Sargent, former chairwoman of the Martha's Vineyard Democratic Party, in Massachusetts. "Trump’s going do it anyway. I don’t know if it’s going to hurt Biden. I think it’s going to hurt everybody."

But some not supporting his campaign have lent credence to the concerns.

In July, Democratic Rep. Alexandria Ocasio-Cortez said that Biden's performance in a debate that month "raised some questions" about his "capacity." The first-term New York lawmaker has endorsed Sanders for the Democratic nomination.

In September, former Housing and Urban Development Secretary Julian Castro questioned Biden's memory during a debate exchange over their healthcare plans.

"Are you forgetting what you said two minutes ago?" said Castro, then a 2020 rival of Biden.

Former 2020 contender Cory Booker backed up Castro in a post-debate interview that night, saying one "has to wonder" about Biden.

"I think that we are at a tough point right now because there’s a lot of people who are concerned about Joe Biden’s ability to carry the ball across the end line without fumbling,” said the New Jersey senator.

Throughout the campaign, Biden's frequent mistakes on things such as gun deaths and even what state he's in have spurred discussion among the electorate. In Iowa and New Hampshire, two states where Biden finished poorly in their respective nominating contests, Democratic voters told the Washington Examiner that Biden's age was a principal concern. Not until recently, however, has it become a potential weapon by the president or Sanders.

"I don’t know why people are questioning why we're bringing this up," Konst said. "This is politics. I want to know if my presidential nominee can take it on stage. They call us snowflakes, and sometimes I see why."

The Republican National Committee and Trump's reelection campaign have begun circulating videos of Biden's verbal blunders, although neither group has gone as far as to say he suffers from any sort of debilitating mental issue.

The age of the current presidential field — Trump is 73, and Sander is 78 — has led to a bipartisan outcry by a number of health professionals. No standard medical tests exist for any individual seeking political office, leaving the details of a candidate's physical well-being completely up to his or her discretion.

In December, former President Barack Obama's longtime doctor, before he reached the White House, told 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," said Dr. David Scheiner after reviewing medical records the Biden campaign disclosed. "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.”