When Gov. Scott Walker of Wisconsin throws his leg across his beloved Harley-Davidson Road King for a celebration of motorcycles and Iowa pork on Saturday, the political symbolism will be as thick as the smoke from the roasting pits.

Mr. Walker, who first broke from the pack of other Republican presidential hopefuls thanks to a speech in Iowa, will be alone among seven declared and likely candidates on a chopper alongside Senator Joni Ernst, a fellow biker and one of Iowa’s most popular Republicans.

The other 2016 candidates will be left in the dust for the initial 28-mile ride of “Joni’s Roast and Ride,” a daylong political fund-raiser.

Mr. Walker, whose early surge in national polls has receded, still enjoys a decisive lead in Iowa, thanks to an unflashy style that resonates with Iowans’ Midwestern sensibilities and to an unusual appeal across a wide ideological swath of Republicans.