An election year already defined by a cascade of national crises descended further into chaos Friday, with President Donald Trump declaring that he's tested positive for the coronavirus after consistently playing down the threat.

Democratic challenger Joe Biden, who spent 90 minutes on stage with Trump in their Tuesday debate, tested negative and moved forward with plans to attend a campaign event in Michigan Friday afternoon.

No one knows exactly what comes next.

Much depends on the extent of Trump's symptoms, but, at the least, 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 in a quarantine, denying him the large public rallies that fuel his campaign just a month before the election.

More broadly, the stunning development 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.

“It's a reminder that the American presidency is bigger than any one person, given the reach and depth this news has,” said Karen Finney, a Democratic consultant and top adviser to Hillary Clinton's 2016 campaign.

Biden and other Democratic officeholders focused largely on Trump's health in public statements, 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.

Republican National Committee Chair Ronna McDaniel has 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.

“Trump's main advantages, including incumbency, have been removed. Rallies, his main vehicle for mobilizing his base, will no longer be possible.