CHICAGO—The scene at the McCormick Place convention center was like a rock concert. Crowds descended on this gargantuan place by the Lake Michigan shore, many of them outwardly giddy about their luck in getting a ticket. A space with restaurants and bars in an adjacent Hyatt hotel was packed beforehand, with people eating, drinking, and laughing. A makeshift souvenir shop with unlicensed merchandise popped up near the entrance to the center. And it wasn't hard to spot hundreds of people wearing shirts from the last tour—or, in this case, political campaign.