INDEPENDENCE, Iowa—Beto O’Rourke is trying to revive his barely four-month-old presidential campaign—again.

After a shaky debate performance—the first opportunity the crowded field of Democratic candidates had to introduce themselves on a national stage—the former Texas congressman made his sixth trip to Iowa for Fourth of July week, driving 540 miles in an RV with his wife and three children. At carnivals, house parties, a baseball game and an Independence Day parade here, Mr. O’Rourke, who tried more national media appearances...