“It angers me when they refuse to adhere to the directions about keeping their masks on,” Ms. Watson Coleman, a lung cancer survivor who will turn 76 next month, said in an interview. “It comes off to me as arrogance and defiance. And you can be both, but not at the expense of someone else.”

Ms. Watson Coleman said after taking a rapid antigen test on Monday, she was isolating and awaiting the results of a more accurate laboratory P.C.R. test. She began to feel symptomatic within the last 24 hours, and was experiencing “mild, cold-like symptoms,” including a cough and a raspy sore throat.

The scene that unfolded on Wednesday in that one secure room — where an offer of masks from Representative Lisa Blunt Rochester, Democrat of Delaware, was rejected by a group of Republicans — is emblematic of the challenge that has dogged Capitol Hill’s disorderly response to the pandemic.