DES MOINES — Alarmed by Newt Gingrich’s decline in the polls in the face of a fusillade of negative advertisements, some senior aides and grass-roots supporters have suggested that he reverse course and fight fire with fire, fearing for the future of his candidacy if he does not. Mr. Gingrich’s vow to stay positive draws applause at every stop of his winding Iowa bus tour, but polls raise questions about that strategy as attack ads against him from his opponents’ campaigns and independent groups have seeped into voters’ minds. New polls of Iowa Republicans this week suggest that Mr. Gingrich...