Senator Rand Paul has tested positive for Covid-19, becoming the first member of the Senate to report a case of the coronavirus.
He said in a tweet that he was feeling fine and was in quarantine.
Dr Paul said he did not have symptoms and was tested out of caution due to his extensive travel and events.
He said he was not aware of any direct contact with any infected person.
Dr Paul was among eight Senate Republicans who voted against a House-passed bill last week that provided more than 100 billion dollars (£85.8 billion) to boost testing for the coronavirus and guarantee paid sick leave for millions of workers.
He also was only Republican senator who opposed an earlier bill authorising 8.3 billion dollars (£7.1 billion) for initial response to the coronavirus.
The senator was on Capitol Hill on Wednesday afternoon, which was the last time the Senate held floor votes, including on one of his amendments.
While Senate Republicans have lunched together as a group most days since, it is unclear if Dr Paul was among them.
John Thune, the second-ranking Republican senator, said on the Senate floor on Sunday that politicians will consult with the attending physician at the Capitol about those senators who have been in contact with Dr Paul.
Two House members, Republicans Mario Diaz Balart and Ben McAdams, have tested positive.
The Senate was in session on Sunday seeking a bipartisan response to the pandemic.
If approved, the Bill would be the third measure Congress has approved in response to the coronavirus this month.