As the primary calendar turns to Connecticut, the Clinton campaign has enlisted a powerful new surrogate who has cut both a 30-second and 60-second ad for the campaign: a daughter of Dawn Hochsprung, the principal killed at Sandy Hook Elementary School.

On Screen

Slides of a wedding day scroll past — a bride in her white dress, smiling as she poses for a picture in sneakers. But the joyous day is quickly shrouded, somber piano chords ringing in the background. “My wedding was one of the last things I planned with my mom,” says Erica Smegielski, Ms. Hochsprung’s daughter.

The camera travels back, for a brief moment, to the scene outside Sandy Hook school, ambulances waiting outside. The text from a New York Daily News article detailing Ms. Hochsprung’s heroic efforts overlays an image of a vigil held outside the school, as her daughter says, “No one is fighting harder to reform our gun laws than Hillary Clinton.”

Scenes of Mrs. Clinton speaking from the campaign trail, including her gun violence town hall in October 2015, and a headline from The Wall Street Journal — “Hillary Clinton Challenges the Gun Lobby” — give evidence to Ms. Smegielski’s statements.