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.
“This test came back positive,” Ben Voelkel, a spokesman for the senator, said in a statement. “Senator Johnson feels healthy and is not experiencing symptoms. He will remain isolated until given the all-clear by his doctor.”
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 49 Democrats and the two dissenting Republicans.
But 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. 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.
The president’s illness forced him off the campaign trail with just 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.