WASHINGTON — President Trump’s announcement on Friday that he had contracted the coronavirus upended the presidential race in an instant, leaving both sides to confront a wrenching set of strategic choices and unexpected questions that will help shape the final month before Election Day.
As the president boarded Marine One to fly to the Walter Reed National Military Medical Center for treatment, his aides announced that they were suspending his campaign events and those of his family members, who are his most ubiquitous surrogates. Privately, his top advisers expressed shock at the turn of events and hope that Mr. Trump’s symptoms would remain mild and he could at least begin appearing on television next week.
At the same time, Joseph R. Biden Jr., Mr. Trump’s Democratic rival, disclosed that he had tested negative for the virus and continued to campaign, beginning with a campaign trip on Friday to Michigan.
With Mr. Biden already leading in the polls, and Mr. Trump’s electoral prospects dependent on his ability to campaign, the president has little time to change the trajectory of the race. The fate of his re-election bid increasingly seemed to hinge on his own health — and whether he will able to overcome the disease and persuade voters to give him another four years.
The split-screen between the candidates on Friday represented a striking reversal from the last few months, during which Mr. Trump pushed on with his rallies and belittled Mr. Biden for adhering to health protocols and running a “basement campaign.”
The former vice president was careful to avoid anything that could be perceived as exploiting the situation Friday; at an appearance in Grand Rapids, Mich., he did not criticize Mr. Trump for his handling of the virus, and closed his remarks by calling on God to “protect the first family, and every family that is dealing with this virus.”
Mr. Biden’s campaign also moved to take down negative television commercials Friday that lashed Mr. Trump for his handling of the virus, according to a Democratic official familiar with the ad traffic. And Mr. Biden’s campaign manager, Jennifer O’Malley Dillon, emailed the entire campaign urging its members to “refrain from posting about the situation on social media.”
Mr. Biden’s aides said he had no plans to step away from his travels — at least for now.
The president’s illness is certain to keep the coronavirus pandemic front and center in the remaining weeks before the election, a development that would appear to favor Mr. Biden, whose campaign message is focused on criticism of Mr. Trump’s stewardship of the deadly disease.
In the White House, advisers to the president acknowledged that the positive test would remind voters of how dismissive Mr. Trump had been about the virus, not only with the neglect of his own safety but also in his overly rosy assessments about a pandemic that has killed more than 208,000 Americans. Mr. Trump’s recklessness, one adviser acknowledged, amounted to a political “disaster.”
As it became clear late Friday that a number of attendees at last week’s Rose Garden announcement of Judge Amy Coney Barrett’s Supreme Court nomination had also tested positive, the White House was also confronting accusations they had hosted a so-called “super spreader” event.
For all the drama 2020 has delivered, the presidential race has been largely impervious to even momentous events, whether it was impeachment, the virus, unrest over racism and severe economic distress. Mr. Biden has enjoyed a steady lead in the polls since effectively claiming the nomination in April.
But an incumbent president testing positive for a potentially deadly disease is of a greater order of magnitude.
Republicans worried on Friday that Mr. Trump would have to remain in the hospital for a significant period of time, imagery they fear would be damaging at a moment when millions of Americans are already voting.
G.O.P. officials were also concerned that a race with very few undecided voters would freeze in place. Multiple party strategists said their polling in the two nights after the presidential debate had revealed substantial slippage, and not just at the top of the ticket.
“This limits Trump’s opportunity to turn this thing around and drive a winning message,” said Terry Sullivan, a Republican consultant. “He’s lost any ability to control the narrative.”
Should the final weeks of the campaign be dominated by the coronavirus, Mr. Trump’s challenge will be intensified by his casual approach to the disease and its deadliness.
The president spent months disregarding and mocking the basic precautions, such as wearing a mask, that his health advisers were urging Americans to take to protect themselves.
Still, few Democrats had any degree of confidence on how the final weeks of the race would play out.
Representative Dina Titus of Nevada said Mr. Biden should proceed. “I don’t see why he should quit campaigning unless something really bad happens,” Ms. Titus said. “And then all bets are off.”
What some Democrats feared, and Republicans hoped, is that there would be a rallying around Mr. Trump and he would garner sympathy from voters. Yet even the most optimistic Republican allowed that those sentiments wouldn’t automatically translate into votes.
At the very least, Republicans said they hoped Mr. Trump’s illness would prompt him to refrain from the inflammatory rhetoric that has alienated many voters and make the election less of a referendum on his behavior.
“Peace and calm helps him,” said Alex Castellanos, a longtime Republican strategist. “He is the polarizing element, not the direction he would like to take the country.”
Mr. Trump’s political fortunes will depend in large part on the severity of his illness. Other world leaders, including Prime Minister Boris Johnson of Britain, have been sickened by the virus and returned to lead their countries.
The 74-year-old president is older than his counterparts who have contracted Covid-19, however, and they were not on the ballot when they tested positive.
In any event, the effect of even seemingly cataclysmic events on the race are hard to predict. After all, when the “Access Hollywood” video emerged just weeks before the election in 2016, it was widely thought that Mr. Trump’s boast of grabbing women’s genitals would effectively end his chances of winning.
Democratic lawmakers on Friday urged Mr. Biden to remain on the campaign trail and tailor his remarks to reflect the seriousness of the moment.
“It’s proof that we need to be vigilant and we need mature leadership,” said Representative Tim Ryan of Ohio. “He doesn’t even need to bring up Trump by name, just say it’s very serious, even the president can get it.”
While Biden aides are being careful not to appear publicly insensitive, they suspect that Mr. Trump had already contracted the virus by the time of his caustic performance in Tuesday’s debate, a campaign official said Friday. And they are angry that members of Mr. Trump’s family refused to wear masks in the debate hall, and appeared to rebuff the efforts of an employee of the Cleveland Clinic to get them to wear one, the official said.
It is almost certain that the remaining two debates between Mr. Trump and Mr. Biden will be affected. The next one is scheduled for less than two weeks from now, on Oct. 15, and the president may be isolated until then.
The nature of the campaign will be disrupted as well. And after having gone forward with the large rallies he craves, despite rules against large gatherings in many states, Mr. Trump will not be able to leave Washington during a final, crucial stretch of the campaign.
Moreover, one of his central arguments against Mr. Biden, that the 77-year-old former vice president is enfeebled and unfit to lead the country, has now been undermined by questions about the president’s own health.
“Trump is now in the position of becoming exhibit No. 1 for the failure of his leadership on coronavirus, and he runs the risk that his supporters will feel misled by his dismissiveness of the virus and the need for precautions,” said Geoff Garin, a Democratic pollster.
The president was already lagging in the polls in part because of his difficulties with older voters, a constituency that leans Republican but is also at the highest risk from the virus.
Some of Mr. Trump’s aides began the day Friday discussing ways for him to be seen by the public. But it became clear by the afternoon that was not possible, and they released a statement from his doctor acknowledging he was fatigued and was taking an experimental antibody cocktail.
In private conversations, members of his staff were also candid that the president had some underlying conditions that could make him more susceptible to a severe bout of the virus.
No modern president has publicly endured a health crisis this close to a re-election attempt. Ronald Reagan was shot and convalesced in 1981, just over two months after he was first sworn in. And Dwight D. Eisenhower suffered a heart attack while in office, but it was more than a year before he faced the voters for a second time.
Some Republicans hoped his ill-fated June rally in Tulsa, Okla., when he couldn’t come close to filling the arena and some of his own staff members got the virus, would serve as a wake-up call.
But while the event put an end to his rallies for a period, it did not make Mr. Trump more sober about the threat of the virus.
The president restarted the rallies during the Democratic convention in August. The events have been mostly, but not always, outdoors, often in hangars at smaller airfields. Yet his supporters, journalists, White House staff members, security workers and others are around one another for hours at the rallies. And many of those who attend, including Mr. Trump and members of his staff, have not worn masks.
Katie Glueck and Matt Stevens contributed reporting.