In an Aug. 26 news conference, Mrs. Clinton said she understood why people had questions about the email arrangement, which she said came about as a matter of convenience so she could carry a single mobile device. She said she took responsibility for the decision to use the private server and said it would have been better to have used a private email only for personal matters and an official one for work.

Last week, Mrs. Clinton’s aides showed a video of that news conference to a New Hampshire focus group of independents and Democrats, according to a Democrat briefed on the focus group whose account was confirmed by a person in her campaign. Participants said they wanted to hear more from Mrs. Clinton about the issue.

The focus group also showed that the email issue was drowning out nearly everything else that Mrs. Clinton was hoping to communicate to voters — something Mrs. Clinton and her husband have complained about to friends.