DES MOINES, Iowa — In a presidential race seemingly frozen in place for months, the advantage has shifted toward President Barack Obama after a series of miscues by Mitt Romney, punctuated by the Republican challenger's comments about people who pay no income tax. Despite a continuing gray economic sky and unrest in the Mideast, the president has edged ahead of Romney in polls in some of the most competitive states, including Iowa and Virginia, and forced Romney to redouble efforts in Florida and Ohio, without which he has little chance of becoming president. With about six weeks left before Election...