Joe Biden stepped up his criticism of President Trump on Monday, challenging his calls not to fear the deadly coronavirus and admonishing him to regulate the use of face masks, as Trump returned to the White House after being hospitalized for covid-19.

Campaigning in Miami, the Democratic presidential candidate signaled an intent to advance a more forceful critique of the president, just days after dialing back his attacks while Trump battled his illness at Walter Reed National Military Medical Center and his prognosis was clouded in uncertainty.

Although Trump was discharged on Monday, there remained many questions about his recovery and outlook, suggesting that Biden may modulate his pitch yet again. The fluidity surrounding the president’s health has added a new challenge for Biden. As he presses ahead with his campaign with just four weeks left until the election, Biden is facing dueling pressures from aides and allies to appear sensitive to his rival’s condition and to present an aggressive closing argument against a second Trump term.

“I was glad to see the president speaking in recorded videos over the weekend,” Biden said in a speech Monday afternoon in the Little Havana neighborhood of Miami. “I would ask him to do this: Listen to the scientists! Support masks! Support a mask mandate nationwide. Require a mask in every federal building, facility in their state. Urge every governor in America to do the same. We know it saves lives.”

Seeking to sharpen the contrast hours later, Biden posted a brief split-screen video on Instagram and Twitter of Trump taking off his mask after he returned to the White House alongside the former vice president putting one on. “Masks save lives,” the caption read.

And in a town hall hosted by NBC News, Biden responded to Trump’s latest attempt to play down the virus that has killed more than 200,000 people in the United States. “I hope no one walks away with the message thinking that it’s not a problem. It’s a serious problem. It’s an international pandemic and we have 4 percent of the population and 20 percent of the deaths,” he said.

The comments served as a blunt rebuke to Trump’s social media missive instructing Americans, “Don’t be afraid of Covid. Don’t let it dominate your life.”

Biden also said he was not surprised Trump contracted the virus. “Anybody who contracts the virus by essentially saying, ‘Masks don’t matter, social distancing doesn’t matter,’ I think is responsible for what happens to them,” he said.

Biden campaign officials said Monday that Trump’s illness is not prompting them to deviate from plans for the former vice president to hit the campaign trail this week, viewing his appearances as opportunities to showcase the safety measures he has practiced, in contrast to Trump. He plans to campaign in Gettysburg, Pa., on Tuesday and Arizona on Thursday.

Biden has frequently worn face masks in public while Trump has mocked his opponent’s mask use. The Democratic nominee has also limited attendance at his events, while Trump has held large rallies at odds with the advice of public health experts.

One Biden adviser said the plan was for the Democrat to be a “walking PSA” for taking precautions against the virus. As he delivered remarks in Miami on Monday, Biden kept his face mask on, an added — and highly visible — layer of safety. While he fielded questions from reporters on a tarmac in Delaware, Biden’s wife Jill pulled him back out of concern that he was getting too close to the media.

In the NBC News town hall, Biden made one of his most impassioned pleas yet for mask use. “What is this macho thing — I’m not going to wear a mask,” he asked, appearing to call out the president. Trump has repeatedly flouted the advice of public health experts who have urged Americans to don protective masks during the pandemic.

Biden campaign officials increasingly feel that the basic contours of the election are cementing, and that Trump has little time left to shift the focus away from anger with his handling of he pandemic and rebound in key states where polls show him trailing. This belief has informed their strategy, prompting them not to make sudden shifts, even as Trump’s illness has upended the race.

“If you work with concrete, there’s the setting phase and the curing phase,” said the Biden adviser, who spoke on the condition of anonymity to be candid. “I’m not sure we’re not in the curing phase.”

At the same time, Biden has been forced to make some course corrections in the wake of Trump’s illness that have caused some awkwardness and raised questions about the future direction of the campaign. Campaign officials have reconsidered their advertising strategy and confronted questions about fundraising.

The latest move was taking a more hostile tone against the president on Monday, as Trump vowed to return to the campaign trail soon and renewed his attacks on Democrats.

“Now that President Trump is busy tweeting campaign messages, I would ask him to do this: Listen to the scientists,” Biden tweeted Monday. In Miami, he warned that the pandemic was “far from over,” even as he wished Trump a swift recovery.

Right after Trump’s diagnosis and hospitalization last week, Biden struck a more careful posture, pulling down ads attacking his rival and making a point of wishing him a speedy recovery even as he criticized the president.

On Saturday, Biden said he was “in a little bit of a spot here because I don’t want to be attacking the president and the first lady now, because they’ve now contracted the coronavirus. Jill and I pray for their quick recovery.” On Twitter the previous day, he sought to strike a unifying tone, declaring, “This cannot be a partisan moment.”

But Biden also criticized the president for not doing more to provide protective equipment to workers most vulnerable to contracting the virus. The idea, advisers said, was to focus on the contrast on policy and not make things personal.

He and his aides also sent differing signals about their approach to the upcoming debate with Trump, which has been cast into uncertainty because of the president’s illness. Biden on Monday told reporters that he would defer to the health experts.

“Listen to the science,” Biden said, when asked what precautions he wants if Trump moves ahead with plans to participate in next week’s debate. “If scientists say that it’s safe ... then I think that’s fine.”

Yet on Sunday, one of his senior advisers was more conclusive, stating unequivocally that Biden would be there.

“We’re hoping that President Trump can participate. We hope — we’re hoping that he’s medically able to participate. And, you know, that’s up to his doctors to clear him,” senior adviser Symone Sanders said on CNN’s “State of the Union.” “But Joe Biden will be at that debate.”

Several hours after that interview aired, senior Biden adviser Anita Dunn offered a slightly different view. “If the president is not cleared at that point or if he is not healthy at that point, then there’s not going to be a debate,” Dunn said, adding that Biden is prepared and eager to go forward with it if possible.

Biden said he was not concerned about contracting the coronavirus even after sharing a stage with Trump last Tuesday for a prolonged period. He has tested negative several times since Friday.

But health experts have said that negative tests are not proof that people with possible exposures to the virus don’t have it. And the Centers for Disease Control and Prevention said Monday that infection can spread through exposure to smaller virus-containing droplets and particles that can remain in the air over long distances and time.

“I’ve been fastidious about the social distancing,” said Biden, who told NBC’s Lester Holt that he and Trump “never got closer than you and I are right now.” He said it was “a little disconcerting” to look out and see Trump’s entourage largely unmasked in the audience.

Annie Linskey contributed to this report.