“This cannot be a partisan moment. It must be an American moment. We have to come together as a nation," Biden declared at a speech in Grand Rapids, warning that the virus “is not going away automatically.” While Biden vowed to continue his cautious approach to campaigning during the pandemic, the president's diagnosis injected even greater uncertainty into an election already plagued by crises that have exploded under Trump's watch: the pandemic, devastating economic fallout and sweeping civil unrest. With millions of Americans already voting, the country on Friday entered uncharted territory that threatened to rattle global markets and political debates around the world.

The development focuses the campaign right where Biden has put his emphasis for months — and where Republicans don't want it: on Trump's uneven response to a pandemic that has killed more than 205,000 people in the U.S. And for the short term, it's grounded Trump under quarantine at Walter Reed National Military Medical Center, denying him the large public rallies that fuel his campaign just a month before the election. Biden and other Democratic officeholders wished Trump well in the wake of his diagnosis, although some could not help but admonish the Republican president, who openly ignored his own administration's social safety recommendations for much of the year.

“Going into crowds unmasked and all the rest was sort of a brazen invitation for this to happen,” House Speaker Nancy Pelosi said on MSNBC. The White House reported Friday evening that Trump will spend “a few days” at the military hospital; the president's doctor reported that Trump was “fatigued” and had been injected with an experimental antibody drug combination still in clinical trials.

His campaign announced that all of Trump's scheduled campaign events were being moved online or temporarily postponed. Trump's family, a steady presence on the campaign trail, was also grounded. Trump campaign manager Bill Stepien and Republican National Committee Chair Ronna McDaniel have tested positive for the virus as well.

But Vice President Mike Pence, who has tested negative, will attend his campaign events as planned. Other world leaders, including Britain's Prime Minister Boris Johnson, have contracted the virus and made full recoveries. But strategists in both parties acknowledged the timing is bad.

Millions of Americans have already begun voting in several key states, and tens of millions more will receive absentee mail-in ballots or begin in-person early voting in the coming weeks. “Trump's main advantages, including incumbency, have been removed. Rallies, his main vehicle for mobilizing his base, will no longer be possible. Fly-bys with Air Force One as a backdrop are gone,” said Republican strategist Rick Tyler, a frequent Trump critic.

He said that Trump's infection also “fundamentally undercuts his entire campaign strategy, which was to ignore the pandemic and make unsubstantiated claims that we've turned the corner and are making an economic comeback.” Biden, meanwhile, moved to take down his ads attacking Trump, according to deputy campaign manager Kate Bedingfield, who noted that the campaign would continue running positive ads. The decision was made before news surfaced that Trump would be moved to a military hospital. Otherwise, Biden is not expected to alter his approach to the campaign significantly as Trump recovers.

The Democratic nominee has been much more cautious on the campaign trail than Trump. Having spent much of the spring and summer avoiding crowds, Biden has held far fewer public events since returning to the campaign trail last month — all of them with small crowds, if any, following social distancing guidelines. Only on Thursday did Biden's campaign announce that it would resume door-to-door canvassing in addition to its phone and digital outreach to voters.

Biden traveled from Delaware to Michigan on Friday afternoon for a campaign event, while Jill Biden was attending a separate event in New Hampshire. Biden's running mate, Kamala Harris, made her previously scheduled trip to Las Vegas as well. The campaign confirmed Biden, his wife and Harris all tested negative for the virus.

“This is not a matter of politics. It's a bracing reminder to all of us,” Biden said in Grand Rapids, calling for a nationwide mask mandate as he spoke wearing a surgical mask. “We have to take this virus seriously. Trump now faces tremendous pressure to adjust his rhetoric and campaign tactics after spending much of the year downplaying the severity of the virus and repeatedly declaring COVID-19 would “disappear.” As recently as Tuesday, Trump ridiculed Biden on national television for his cautious approach.

“I put a mask on when I think I need it,” Trump said during the debate. “I don't wear masks like him. Every time you see him he's got a mask. He could be speaking 200 feet away, and he shows up with the biggest mask I've ever seen.” Two additional debates are scheduled for Oct. 15 and Oct. 22. The Commission on Presidential Debates has not yet commented on any changes in the debate schedule or health protocols, but has confirmed that next week's vice presidential debate is on as scheduled.