In a complicated political season, nobody has more complications to worry about than the half-dozen Republican senators seeking re-election in tight, swing-state contests. Those senators–hailing from Illinois, New Hampshire, Ohio, Pennsylvania, Wisconsin and North Carolina—are being buffeted by big forces largely out of their control as they try to hang onto their seats. Their fate will determine whether Republicans keep control of the Senate, where they currently have a 54-46 advantage. The predicament of these GOP senators comes into clearer focus in the latest Wall Street Journal/NBC News poll, completed just last week, which helps crystallize three big complications they...