Under normal circumstances, Joseph R. Biden Jr. might have delivered a speech on race in America on Sunday, covered by a press corps following him around the country. He might have visited Minneapolis or another city torn by violence. He might have summoned reporters to the front of his plane to critique President Trump’s leadership of a nation in crisis.

But at a moment that is emerging as a critical test for both Mr. Biden and Mr. Trump, the presumptive Democratic candidate for president is constrained by the limitations of a pandemic that has confined him to his home in Wilmington, Del., for the past three months.

Until now, Mr. Biden has sought to make the best of his circumstances with remote speeches, fund-raisers and interviews over Zoom, and digital advertisements.

Yet the president’s struggles are providing Mr. Biden with an opportunity to show an anxious nation how he might lead during these twin crises of civil unrest and a health emergency that has killed more than 100,000 Americans. The question is whether he can do this largely from the confines of his home or whether the moment has come to join other Americans — Mr. Trump among them — who have begun venturing into the world, to make his case to voters to oust a sitting president.

Mr. Biden may be thinking that moment has come. On Sunday, his announced schedule consisted only of delivering recorded remarks to a virtual rally in Maryland. But late in the day the former vice president made an unannounced tour of businesses in downtown Wilmington damaged during protests. On Monday he will meet with community leaders there.

Presidential campaigns can be defined by moments like this. Bill Clinton made a dramatic trip to Los Angeles, its streets empty and smoke still lingering in the air, after the riots of 1992. It was a high-risk move, providing images of Mr. Clinton touring the ruins of strip malls in Koreatown, playing basketball in what was called South-Central, and speaking at the First African Methodist Episcopal Church. He went on to defeat President George Bush six months later.

A visit by Mr. Biden to Minneapolis, or any of the other many American cities that were convulsed by protests and riots over the weekend, would be complicated because of the Covid-19 pandemic. Mr. Biden has broken his quarantine only twice for public appearances: the trip downtown on Sunday, and a visit on Memorial Day to lay a wreath at a veterans’ memorial.

But as tensions have mounted since the death of a black man, George Floyd, in the custody of Minneapolis police officers last Monday, Mr. Biden has raised, at least somewhat, his national profile — to the relief of some Democrats who fear he has had slipped off the radar screen in the midst of an epidemic that has riveted the nation.

He delivered remarks about the plight of black Americans on Friday, the day after Minneapolis was roiled by demonstrations. He issued a statement late Saturday night as looting and fires swept American cities, supporting the protesters while condemning the violence. He called Mr. Floyd’s family.

“He’s been present,” said Lis Smith, who was a senior adviser to the presidential campaign of Pete Buttigieg, the former mayor of South Bend, Ind. “I don’t think it matters to people whether he’s giving an address from his home or from behind a podium. No one in the cities facing unrest is sitting around thinking that the thing that could make this all better is having a presidential candidate in town stoking the fires.”

The challenges of this moment would be daunting for any candidate. And even Democrats who admire Mr. Biden have long worried about his political skills and fortitude. He is 77 and given to verbal missteps.

Can Mr. Biden give the nation hope and reassurance, and demonstrate how he might lead the country through a period of racial turmoil? Can he address a country’s fears in the midst of a pandemic that has sickened 1.7 million Americans and gutted the economy?

And can he navigate what has historically been a fault line for the Democratic Party, speaking at once to aggrieved African-Americans and others enraged by the death of Mr. Floyd while not alienating white blue-collar voters who prioritize law and order and have fled the Democratic Party for Mr. Trump?

For Mr. Biden, the risks of staying in political isolation are likely to escalate as these twin crises play out. He is competing for attention not only with Mr. Trump but also with Democratic leaders in Congress and mayors and governors. CNN did not provide live coverage of his remarks about Mr. Floyd on Friday as its correspondents waited for Mr. Trump to show up in the Rose Garden to talk about China and the World Health Organization.

And while Mr. Biden has self-quarantined in deference to stay-at-home directives by health officials — and to his own vulnerability to the disease, given his age — Mr. Trump has made it clear that he wants to return to something approaching a normal campaign and will resume in-person fund-raisers in June.

Still, for the time being, there are some benefits to Mr. Biden’s surgical approach to the campaign. Less is sometimes more in politics, particularly for a candidate who does not have the platform of public office.

Mr. Biden’s short but stark appearance on Friday showed that when he does step onto a large platform, his appearances can be powerful and effective. In this case, it set up a sharp contrast with Mr. Trump’s terse news conference at the White House, where he initially made no mention of the events engulfing the nation.

Mr. Biden was able to draw another contrast with Mr. Trump last Wednesday night as he paid tribute in a video to the 100,000 Americans who have died of Covid-19. Mr. Trump has struggled to seem empathetic as the nation confronted this grim milestone.

And as this long spring churns on, Mr. Biden’s confinement has freed him of the trappings of the modern campaign, its days packed with expensive and time-consuming rallies and chartered jets, which often seem a throwback to a predigital era. It has spared Mr. Biden the 14-hour days that can be exhausting even for a candidate half his age.

He is able to spend far less money by staying off the road, no small matter as he confronts an incumbent with a $187 million general election cash advantage. His isolation has allowed him to accelerate hiring that was constrained during the primaries by the campaign’s money shortage.

The virus crisis has taken him a little out of the spotlight — probably not a bad thing, given his propensity for saying the wrong thing — and focused attention on Mr. Trump as he struggles to manage a deadly pandemic and civil unrest across the nation.

“It’s fine — it’s more than fine,” said Rahm Emanuel, the former Democratic mayor of Chicago who was chief of staff to former President Barack Obama. “This is a race of Trump versus Trump, and Trump is losing.”

Scott Reed, a Republican consultant who managed the 1996 presidential campaign of Bob Dole, said Mr. Biden’s situation was reminiscent of the challenge Mr. Dole faced after capturing the nomination in April 1996 and then having to fill time, and attract attention, for four months until the convention. But this, he said, is playing more to Mr. Biden’s advantage.

“The basement strategy may become the new front porch strategy,” Mr. Reed said. “He may have found a new way to skin the cat.”

That is not to say that Mr. Biden would have chosen to spend the months leading to the convention in seclusion at his home rather than on the campaign trail. The former vice president has considerable work to do to energize liberals and younger voters. But it is far from clear whether he can do that without showing up and engaging those voters face to face. More broadly, Mr. Biden faces the challenge of building enthusiasm for his candidacy, something Mr. Trump enjoys from his devoted base.

Potentially most damaging, the pandemic is almost certain to impede the ability of surrogates and campaign volunteers to go door to door building up support.

“The virus is really hurting the ability of Joe Biden — his entire campaign and people outside his campaign — to mobilize,” said Jeff Weaver, a senior adviser to Senator Bernie Sanders, whom Mr. Biden defeated in the primaries. “Candidates for public office should be out among the people. Biden can’t be there. Surrogates can’t be there. You can’t have people knocking on doors.”

A critical question now is whether Mr. Biden and his campaign have the agility to make the most of this moment. Fund-raising was always going to be a big priority for Mr. Biden during this interregnum. The fact that he is not traveling should spare the campaign the overhead of the lavish events, with food and open bars, that donors expect these days.

“There are a lot of upsides for him,” Ms. Smith said. “The way we run presidential campaigns in 2020 is inefficient, outdated and expensive.”

For the past four years, Mr. Trump has been an inescapable public presence, from Twitter to television to rallies. By contrast, two former presidents — George W. Bush and Mr. Obama — made rare forays onto the public stage in recent weeks. Mr. Bush posted a video appealing for partisan unity around the coronavirus, and Mr. Obama spoke to graduating high school seniors.

That drew a flurry of attention, the same way Mr. Biden’s carefully chosen outings have won him notice over these past 10 days.

The digital technology of quarantine and his pick-your-moments schedule may have carried him through his low-profile spring.

But the campaign seems to have moved to its next chapter, as the nation confronts yet another crisis, and Mr. Trump struggles to find his bearings. For Mr. Biden, this could be an opportunity to step up and demonstrate how he, as president, would fill what many Democrats (and some Republicans) see as a void in leadership in the White House. That might be a hard case to make from a basement.

Stephanie Saul contributed reporting.