DES MOINES, Iowa — Hillary Rodham Clinton denounced Jeb Bush and discussed the rise of the Islamic State. Bernie Sanders held forth on Social Security. But at the Iowa State Fair, where a life-size butter cow rules and fairgoers devour stick-impaled delicacies, it is the candidacy of Donald J. Trump that perhaps best captures the air of raucous excess.

Mr. Trump arrived by helicopter, which touched down on the outskirts of the fairgrounds. His campaign later offered children rides as if the chopper were one of the fair’s amusement park attractions. Then, in his dark blue blazer, Mr. Trump strode through the fairgrounds, parting the crowd in his red “Make America Great” baseball hat, waving to the gathered masses along the way.

The mob of news media and fairgoers following Mr. Trump drifted in a unified blob of camera phones, sweat and spilled beer, past the Ultimate Brisket Bomb booth toward the Agriculture Building where the butter cow rests.