Gavin Newsom, San Francisco's glamorous mayor and up-and-comer in national Democratic politics, is supposed to be a smooth operator. So why on earth would he have an affair with an aide's wife, and then put his re-election chances in that aide's hands? Mindboggling. Alex Tourk, who served as Newsom's deputy chief of staff before becoming campaign manager for his re-election, resigned yesterday after discovering that his wife had an affair a year and a half ago with the mayor. The stupid affair won't wreck Newsom's election, or his schmoozing with San Francisco high society and Silicon Valley tycoons; but it must kneecap any national political future.