Joseph R. Biden Jr. tested negative for the coronavirus, his doctor announced on Friday, hours after President Trump’s positive coronavirus test drastically altered the nature of the race and underscored the health and political risks of seeking to engage voters in a pandemic.
Mr. Biden still intended to travel to Michigan later in the day, where he was previously scheduled to hold campaign events. The trip comes after both Mr. Biden and others who had recently traveled with him were tested for the virus. In a message posted on Twitter, Mr. Biden said the episode offered a reminder to “wear a mask, keep social distance, and wash your hands.”
He also tweeted a message of best wishes to the president and his wife, Melania Trump, who tested positive as well. “Jill and I send our thoughts to President Trump and First Lady Melania Trump for a swift recovery,’’ Mr. Biden wrote. “We will continue to pray for the health and safety of the president and his family.”
Mr. Biden, who refrained from holding in-person events for months as the nation struggled to confront the virus, had only recently — and cautiously — begun to campaign on the ground consistently in key battleground states, amid concerns from local Democrats that he had not been visible enough. Mr. Trump, for his part, held large-scale rallies and often played down the risks of a virus that has killed more than 207,000 people. On Tuesday, the two candidates shared a debate stage in Ohio.
Mr. Trump’s diagnosis again pushed the dangers of the virus to the forefront of the campaign with just one month until Election Day and injected new uncertainty into an already extraordinarily volatile race.
Mr. Biden, 77, was tested on Friday, as were others who attended the presidential debate with him earlier this week, including his wife, Jill Biden. A statement from Mr. Biden’s doctor, Kevin O’Connor, said that Covid-19 “was not detected.” The campaign released the statement.
“I’m happy to report that Jill and I have tested negative for COVID,” Mr. Biden tweeted. “Thank you to everyone for your messages of concern. I hope this serves as a reminder: wear a mask, keep social distance, and wash your hands.’’
Democrats, in the meantime, urged Mr. Biden to continue on the campaign trail.
“This is not a hiatus of the campaign,” said Senator Tim Kaine, Democrat of Virginia, even as he said he expected the campaign to continue to follow “good safety rules.” “We’re in the closing stretch of the campaign and they’ve got to be aggressively campaigning.”
Mr. Biden has for months sought to make the campaign a referendum on Mr. Trump’s stewardship of the virus crisis, and Mr. Trump’s positive test result makes the matter both more urgent and more politically delicate. Some Democrats were urging Mr. Biden to continue to emphasize the virus while remaining respectful of Mr. Trump’s condition.
“He doesn’t even need to bring up Trump by name, just say it’s very serious, even the president can get it,” Representative Tim Ryan of Ohio said.
In a note sent to staff, Mr. Biden’s campaign manager, Jennifer O’Malley Dillon, asked the team to “refrain from posting about the situation on social media unless otherwise directed by your manager,” and promised that “the health and safety of the entire team has been, and will remain, our number one priority.”
As Mr. Trump, 74, held packed rallies over the summer and declined to wear a mask consistently, Mr. Biden had insisted it was more important to model public health recommendations about social distancing. Amid concerns about the health of both the candidate and his supporters, the Biden campaign was run almost entirely virtually.
That calculation began to change around Labor Day as Mr. Biden, under the urging of Democratic officials in swing states, pursued more cautious in-person interactions with voters beyond his home state of Delaware and neighboring Pennsylvania. His campaign said this week that their team would resume in-person canvassing in important battleground states.
In late August, the campaign said that Mr. Biden and his running mate, Senator Kamala Harris, would be tested regularly for the coronavirus, and that staff members who interacted with them would also be tested regularly. The campaign said it would announce publicly if Mr. Biden or Ms. Harris ever had a confirmed positive case.
On Friday, the campaign had released no official statement until past noon, hours after Mr. Trump’s test result was made public. Mr. Biden had still not left Delaware, so his first event in Michigan, scheduled for 1:20 p.m. in Grand Rapids, appeared likely to be delayed.
Ms. Harris tweeted that she was keeping the president, the first lady “and the entire Trump family in our thoughts.” Ms. Harris was tested on Thursday for the coronavirus and her test was negative, according to a Harris aide.
She was scheduled to campaign in Nevada on Friday.
Representative Debbie Dingell, Democrat of Michigan, noted that Mr. Biden has sought to be careful in how he campaigns, wearing a mask and aiming to follow social distancing guidelines.
“He did what the science and doctors told us we need to do,” she said. “We’ve all got to assess about where we are, how widespread, who’s been impacted, and go from there.”
Jonathan Martin contributed reporting.