The president regularly hosts Republican senators at the White House, including for the introduction of Supreme Court nominee Amy Coney Barrett. At these events, these lawmakers and much of the crowd are maskless. Senators were tested prior to the Barrett announcement, according a person with knowledge of the event, but testing does not always pick up the disease.

While some influential Senate Republicans have stumped for a comprehensive testing program in the Capitol, leaders in both parties have rejected it. That makes it difficult to know at the moment whether the spread of coronavirus has breached the Capitol, particularly after the Senate adjourned for the weekend and sent senators back to their home states across the country.