URBANDALE, Iowa — On the evening of the Iowa caucuses, Joe Burns, a 71-year-old convenience store manager here, came to Walnut Hills Elementary School in sharply contested Dallas County with the intention of making up his mind. Burns and his wife, Mary, who are among Iowa’s vast ranks of undecided voters, followed a line past lost-and-found boxes filled with mittens and scarves and a mural that read “It’s About Respect, Respect Iowa.” In the school’s cafeteria, the county recorder, wearing a blue Mitt Romney sweater, registered voters and handed Burns a slip of paper with the names of the competing candidates.