Planting one on the forehead of mama’s darling used to be the surest route to public office. These days, it may be playing with a talk-show’s house band. Presidential candidate Mike Huckabee appeared on Leno last night, and though his explanation of his flat consumption tax might have put half the late-night audience to sleep, he sprung that only after his turn playing bass guitar beside Leno’s bandleader, Kevin Eubanks.Huckabee caught grief from his rivals for deserting the field of battle in Iowa at the 11th hour, but it was likely their panic talking: back in 1992, it was Bill Clinton’s appearance on “The Arsenio Hall Show,” where he brought his sax and sat in on “Heartbreak Hotel,” that sealed the deal with the American public. (Said Arsenio: “It’s nice to see a Democrat blow something besides the election.”) If Huckabee suffers any fallout from his Leno spot, it will be with the unions, who now consider him a scab for crossing the picket lines of the Hollywood writers’ strike.