PORTSMOUTH, N.H. — Hillary Clinton is dropping in the polls here, and her presidential campaign is beset by the continued controversy over her private e-mail server, but she tried to place both issues on the back burner here Saturday in a showy display of what her latest big-name endorser called “pink power.” On a warm and clear day in this historic New England port city, Clinton rallied about 1,300 supporters on the steps of an old schoolhouse with a focus on women’s and family issues.