WASHINGTON — President Trump’s vital signs were “very concerning” over the last day and he is not out of danger, the White House chief of staff said on Saturday, contradicting a rosier picture painted by the president’s doctors on television just minutes before. While the doctors maintained that Mr. Trump was “doing very well” and in “exceptionally good spirits” after his first night in the hospital with the coronavirus, Mark Meadows, the chief of staff, provided a more sober assessment and warned that the next two days would be pivotal in determining the outcome of the illness. “The president’s vitals over the last 24 hours were very concerning and the next 48 hours will be critical in terms of his care,” Mr. Meadows told reporters outside Walter Reed Medical Military Center, where the president was flown on Friday evening and will remain for at least a few days. “We’re still not on a clear path to a full recovery.” Mr. Meadows’s remarks were attributed to a person familiar with the president’s health in a pool report sent to White House journalists in keeping with ground rules that he set for the interview. But a video posted online captured Mr. Meadows approaching the pool reporters outside Walter Reed after the doctors’ televised briefing and asking to speak off the record, making clear who the unnamed source was. The mixed messages, which came even as the virus spread to a widening swath of Mr. Trump’s allies, only exacerbated the confusion and uncertainties surrounding the president’s situation. During their briefing, the doctors refused to provide important details and gave timelines that conflicted with earlier White House accounts, leaving the impression that the president was sick and began treatment earlier than officially reported. Two people close to the White House said in separate interviews with The New York Times that the president had trouble breathing on Friday and that his oxygen level dropped, prompting his doctors to give him supplemental oxygen while at the White House and transfer him to Walter Reed where he could be monitored with better equipment and treated more rapidly in case of trouble. During the televised briefing, Dr. Sean P. Conley, the White House physician, said the president was not currently receiving supplemental oxygen on Saturday but repeatedly declined to say definitively whether he had ever been on oxygen. “None at this moment and yesterday with the team, while we were all here, he was not on oxygen,” he said, seeming to suggest that there was a period on Friday at the White House when he was. Dr. Conley likewise appeared to indicate that the president was first diagnosed with the virus on Wednesday rather than Thursday night when Mr. Trump disclosed that he had tested positive on Twitter. While describing what he said was the president’s progress, he said Mr. Trump was “just 72 hours into the diagnosis now,” which would mean midday on Wednesday. Asked about that, Dr. Conley did not clarify but said that on Thursday afternoon “we repeated testing and, given clinical indications, had a little bit more concern.” Late that night, he said “we got the PCR confirmation that he was” positive. Mr. Trump attended campaign events on both Wednesday night and Thursday without wearing a mask and gathering hundreds of supporters who likewise were not taking precautions against the virus. Dr. Brian Garibaldi, another physician treating the president, also said that Mr. Trump had received an experimental antibody therapy “about 48 hours ago,” which would have been midday Thursday — before the confirmation test Dr. Conley said came back positive that evening and a full day before the White House disclosed the treatment on Friday. The confusion came from a briefing where Dr. Conley and his team offered a relentlessly positive assessment of Mr. Trump’s condition. “This morning the president is doing very well,” Dr. Conley said. “At this time, the team and I are extremely happy with the progress the president has made.” The doctors said Mr. Trump had been free of fever for 24 hours and had blood pressure and heart rates that were normal for him. Asked why he moved Mr. Trump to the hospital, Dr. Conley said, “Because he’s the president of the United States.” Dr. Sean N. Dooley, another physician on the team, said Mr. Trump was feeling optimistic. “He’s in exceptionally good spirits,” Dr. Dooley said. He added that the president told his doctors, “I feel like I could walk out of here today.” Mr. Trump amplified that buoyant tone in a Twitter message on Saturday afternoon. “Doctors, Nurses and ALL at the GREAT Walter Reed Medical Center, and others from likewise incredible institutions who have joined them, are AMAZING!!!” he wrote. “Tremendous progress has been made over the last 6 months in fighting this PLAGUE. With their help, I am feeling well!” Mr. Meadows later tried to walk back his earlier comments. “The president is doing very well,” he told Reuters. “He is up and about and asking for documents to review. The doctors are very pleased with his vital signs. I have met with him on multiple occasions today on a variety of issues.” The outbreak infected a third Republican senator on Saturday as Ron Johnson of Wisconsin reported testing positive as did former Gov. Chris Christie of New Jersey, who helped prepare Mr. Trump for his campaign debate on Tuesday. Other Republicans close to Mr. Trump were being tested and awaiting results as the weekend opened. In the course of barely 24 hours, the president, his wife, his campaign manager, his party chairwoman, his senior adviser, his former counselor and now three Republican senators have all tested positive for the virus, along with several reporters. The White House medical unit was investigating Mr. Trump’s announcement of his nomination of Judge Amy Coney Barrett to the Supreme Court last weekend as a likely source of the virus’s spread. More than a half-dozen people who attended the event on Sept. 26 or were with the president on Air Force One flying to a campaign rally in Pennsylvania later in the evening have now tested positive. While the ceremony itself was outdoors in the Rose Garden, most of the people in attendance other than reporters were not wearing masks or keeping socially distant. A number of people also joined Mr. Trump and Judge Barrett inside the White House for a reception, again without masks and in some cases shaking hands and hugging without evident regard for the dangers of the virus. With the election just 31 days away, White House officials sought on Saturday to project as much of a business-as-usual image as possible, insisting that the president can govern the country from his hospital bed and that there was no need to transfer power to Vice President Mike Pence. Even as doctors hovered over Mr. Trump, his staff on Friday night issued a report on his buy-American drive and announced some minor appointments. On Saturday morning, the White House announced that the president had signed two bills appointing members to the Board of Regents of the Smithsonian Institution. Mr. Trump has been suffering from a cough, congestion, fatigue and a fever, according to people informed about his condition, and some of the symptoms worsened as Friday progressed including the drop in oxygen level that alarmed the president. Plans for him to convalesce at the White House were abruptly scrubbed in favor of sending him to the hospital for what officials said would be a few days. One reason, according to an administration official, was that it would be better for the president to leave while he could still walk on his own power to the helicopter rather than risk getting sicker and having the politically damaging image of needing assistance to be transported to the hospital later. At Walter Reed, doctors put Mr. Trump on remdesivir, an antiviral drug that has hastened the recovery of some coronavirus patients, which came after a variety of other treatments were administered while the president was still at the White House. Mr. Trump received a single 8-gram dose of polyclonal antibody cocktail, an experimental treatment that the White House obtained special permission to try, as well as zinc, vitamin D, an anti-heartburn medicine called famotidine, melatonin and aspirin. In a statement on Friday, Dr. Conley said that Mr. Trump received the antibody cocktail “following PCR-confirmation of the president’s diagnosis.” But if he started it 48 hours earlier, as Dr. Garibaldi said on Saturday, that would mean he had begun treatment by midday on Thursday, before the PCR test that Dr. Conley said came in late Thursday night, and before he flew to New Jersey for a fund-raising event at his golf club. Melania Trump, the first lady, who also is sick, remained at the White House while the rest of the president’s family members have tested negative but were being retested to confirm those results since it can take some time before the virus is detectable. “Thanks to all those who so lovingly have reached out about @realDonaldTrump and the rest of the family,” Donald Trump Jr., the president’s eldest son, tweeted on Saturday morning. “It truly means a lot to us. I tested negative so I’ll give it a few more days out of and abundance of caution and test again and if I’m clear I’ll be back to work asap.” Among those who tested positive on Saturday was Mr. Christie, who was at the Barrett announcement a week earlier. Two of the three senators who have now tested positive for the virus, Mike Lee of Utah and Thom Tillis of North Carolina, were also at the event and serve on the Senate Judiciary Committee that will consider her nomination, further complicating a confirmation drive that was already testing speed limits and a razor-thin majority to get done before the election, as Mr. Trump has insisted. Mr. Johnson, the third senator now infected, was not at the event because he was quarantining from a previous exposure to someone with the virus. He emerged from 14 days of quarantine after testing negative and returned to Washington on Tuesday, his office said, but attended lunch with other Senate Republicans that put him in the same room with Mr. Lee and Mr. Tillis. He was tested again on Friday. If all three Republicans were unable to vote, then Senator Mitch McConnell, the Republican majority leader from Kentucky, would not have enough support to force through confirmation of Judge Barrett. Because two moderate Republican senators, Susan Collins of Maine and Lisa Murkowski of Alaska, have already indicated that they oppose installing a new justice before the election, that would leave Mr. McConnell with 48 Republicans to 47 Democrats and the two dissenting Republicans. But even as Mr. McConnell said on Saturday that the full chamber would not meet this coming week as planned, Republican leaders vowed to push ahead with the confirmation on the hope that the infected senators would not get sick and therefore be able to return in time for a pre-election vote. Senator Lindsey Graham, Republican of South Carolina and chairman of the Judiciary Committee, issued a statement saying the panel would proceed with plans to meet on Oct. 12 to begin considering Judge Barrett’s nomination. But Republicans could need contingency plans depending on whether the three senators who tested positive get sick. One option being discussed was holding the Judiciary Committee vote in the Senate chamber with the infected senators perched in the galleries overlooking the floor, far from their colleagues. Democrats complained about the Republican persistence in the face of the infections. “To proceed at this juncture with a hearing to consider Judge Barrett’s nomination to the Supreme Court threatens the health and safety of all those who are called upon to do the work of this body,” Senator Dianne Feinstein of California and the rest of the committee’s Democrats said in a letter to Mr. Graham. The president’s illness forced him off the campaign trail with only a month until the election on Nov. 3. Mr. Trump’s events have been canceled, as have those of his immediate family for the moment while they confirm negative test results, leaving it to Mr. Pence, who has tested negative for the virus, to pick up the burden of the contest against former Vice President Joseph R. Biden Jr., the Democratic nominee. The Trump campaign announced on Saturday that Mr. Pence would host an unspecified campaign event on Thursday in Peoria, Ariz. “Joe Biden has turned his back on Arizonans by advocating for the far-left’s agenda of harsh regulations and high taxes,” the statement said. Mr. Biden has said he is praying for Mr. Trump’s speedy recovery and his campaign has taken down negative television advertisements assailing the president’s handling of the coronavirus crisis that has killed more than 208,000 people in the United States so far. But the candidate was not backing off his own campaign schedule. Peter Baker reported from Washington, and Maggie Haberman from New York.