He expressed dismay over Trump's response to the coronavirus pandemic, ridiculing him for not doing more sooner, urging him to listen to scientific experts and saying of the President's plan to get Americans back to work by Easter: "What's he talking about?"
"He should stop talking and start listening to the medical experts," Biden said on CNN. "What is going on with this man?"
"He says he's a wartime president - well, God, act like one," he added. "Move. Fast."
"I just can't figure the guy," Biden said on MSNBC. "It's like watching a yo-yo."
The comments prompted relief from some Democrats who had worried Biden was rendering himself almost invisible.
"A week ago I would have said he wasn't present nearly enough. Now, it seems like they're finding their footing and he's taking the right tone," said Julián Castro, the former housing secretary under President Barack Obama and a former Democratic presidential candidate.
Castro, who has not endorsed Biden, added that the former vice-president's moves appear to be a "work in progress."
"He has a lot of experience and Vice-President Joe Biden does reassure people," Castro added. "He should use that - and I think that's why he needs to be more present."
Although Biden has more than four decades of experience as an elected official, he holds no current role that would give him public standing amid the crisis, apart from being the leading candidate in the Democratic presidential race. Senator Bernie Sanders, I, is his sole remaining competitor.
Trump's daily briefings have been widely aired. Among Democrats, the most visible daily appearances have been made by two governors, Andrew Cuomo of New York and Gavin Newsom of California. As Americans seek immediate results, Biden has been left to propose actions that could not be implemented for another 10 months.
In recent days, Biden also has deferred to House Speaker Nancy Pelosi, D, and Senate Minority Leader Chuck Schumer, D, not wanting to do or say anything to unravel delicate negotiations in Congress over measures to help workers and companies.
But that low profile drove Democratic concerns that he was unable to capitalise on Trump's widely panned early response.
"I have some sympathy for him because it's a hard thing. He has no formal responsibility. You can't go out. You can't have events. It's hard," said David Axelrod, a Democratic strategist who helped run Obama's campaign during the economic crisis in 2008. "But I think there are things he could do. You can do things that are more interesting than giving poorly produced quasi-presidential speeches."
Some Democrats have privately told Biden that he has to improve.
"They have to be more creative about the techniques they use in terms of getting him out there and not just rely on speeches," Axelrod said. "And they need to do it well."
Many Democrats pointed to a video featuring Biden adviser Ron Klain - an explainer, complete with a whiteboard, about Trump's coronavirus response and the holes in his arguments - as a viral success. But it also underscored the failings in Biden's own events - speeches that have at times been halting or beset by technical problems.
Other Democrats have organised more seamless appearances. Over the past week, former Texas congressman Beto O'Rourke has used Facebook Live several times to get messages out. Senator Elizabeth Warren headlined two conference calls with activists to gin up support for the priorities she was pushing to include in congressional coronavirus legislation.
Sanders held an online coronavirus town hall yesterday with Representative Pramila Jayapal, D, and health experts. In recent days, he held a similar event that featured congresswomen Alexandria Ocasio-Cortez, D, Ilhan Omar, D, and Rashida Tlaib, D, as well as a round table on the virus and a live-streamed speech on the outbreak. Before that event, Sanders had held seven coronavirus-related events that were seen by 12.7 million viewers. He also raised more than US$2 million for various charities that support those affected by the outbreak.
Biden campaign advisers say they are trying to do more and attempting to get him in front of a camera every day. They also are trying to hone other ideas, such as live-streamed conversations with doctors or with young people, or events highlighting grocery workers and others whose roles have become essential.
One favoured format is the telephone town hall, although its obvious downside is that it does little to drive cable television coverage at a time when many Americans are home watching the news.
The demand by some Democrats that Biden more aggressively seek out attention comes as key blocs of the party are beginning to coalesce around his candidacy. In the past few weeks, as he has won a string of primaries, several of the largest labour unions in the country have backed Biden.
"These two things give him the latitude to be the standard-bearer of a fight for people and for working families," said Randi Weingarten, the president of the American Federation of Teachers, the county's second-largest teachers union. "He now has the latitude to be the standard-bearer, and I think you're going to see that more and more."
Weingarten said that even with that mantle, Biden has had to be careful not to disrupt negotiations on Capitol Hill over a stimulus package. "You had to have Pelosi and Schumer and others in the Congress trying to get to a bipartisan bill. And Biden understood that, and he's actually letting them do that," Weingarten said.
Some Democrats pointed out that Biden has demonstrated he has an enormous well of support among the party's voters, which gives him a wide berth to work out an appropriate response to Trump.
"What that miracle of Super Tuesday showed is there really is a depth of support for Biden in this country, even though he was coming off a year where he was not operating at the top of his game," said Jennifer Palmieri, who was Hillary Clinton's communications director in 2016. "I think he understands he's a vessel for people who have decided, 'You're the guy that we want to take on Trump.' "
"There's a tonne on the line, and he and his team want to put the best effort in," Palmieri added. "I think that he is not in a situation where he has to introduce himself to people . . . everybody knows who he is, they're not going to forget about him."
Like other Americans, Biden has found his own life altered in recent weeks. He is no longer surrounded daily by his advisers, many of whom are now working from home.
Instead, he has been holding two 90-minute conference calls each morning, one with a group of health experts and another with a team of economic advisers.
Many were involved in the Obama Administration's response to the Ebola outbreak in West Africa and, earlier, to the fallout from the 2008 economic collapse.
In a chilling detail that indicates the added anxiety around his age and medical vulnerability, Biden revealed several days ago that when campaign workers occasionally come to his house, they wear gloves and masks. He recently began receiving protection from the Secret Service, and those agents, too, are wearing gloves and masks.
His grandchildren live about 1.5km away and come to his house daily, but they speak to him from the backyard while he sits on the porch.
Biden said that he has not been tested for the coronavirus and that his doctors have told him he does not have any underlying conditions that could increase his risk, beyond his age.
Biden has indicated that he will soon begin narrowing the options for his running mate, whom he has said will be a woman. His initial list holds more than a dozen names, but he said he is attempting to narrow it down to about 11 for more serious vetting. Biden also said he has a list of four black female candidates for the Supreme Court, although he did not reveal their names.
His more immediate concern is Sanders's continued presence. Biden's advisers have been speaking to those close to the senator, trying to find common ground, although Sanders is showing no signs of an imminent departure.
"It's not for me to tell him to drop out," Biden said on The View. "It's up to Bernie what he wants to do."
Even as he ramped up his public presence, however, the coronavirus was never far from mind. Midway through a CNN interview, Biden coughed into his hand. Anchor Jake Tapper gently chided him, reminding him health officials favour coughing into an elbow to prevent spreading germs.
"Fortunately I'm alone in my home," Biden said, before conceding that Tapper was correct.
