Of 31 participants in the group, which gathered on Friday after news of the attacks broke, 26 saw Mrs. Clinton as a stronger potential commander-in-chief than Senator Bernie Sanders of Vermont. “They see her as tough and possessing the right experience,” Chris Kofinis, a Democratic consultant who moderated the focus group, wrote in a memo. “It is fair to say that Sanders was extremely vulnerable on this issue before Paris, and that is even truer now.”

Mr. Kofinis convened the group of young Iowans, aged 18 to 35, which was not sponsored by any candidate or group, in Des Moines, where the debate is taking place. CBS News, the debate sponsor, plans to focus significantly on the implications of the attacks.

The focus group did reveal a potential strength for Mr. Sanders, who trails Mrs. Clinton by double digits in recent Iowa polls: Every one of the participants said Mr. Sanders had a “stronger message.” But the problem for him is that Mrs. Clinton, a former secretary of state, was considered far more electable, by a margin of 27 to 4.