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…

Mr. Walker’s favorability rating among likely Republican caucusgoers in Iowa was higher than any other candidate’s, 67 percent, in a poll conducted at the end of May for The Des Moines Register and Bloomberg Politics (with a five-point sampling error). He enjoyed a solid seven-point lead, an opening he first created with a passionate speech to conservatives in Des Moines in January.