Mitt Romney is heading into his nominating convention with his advisers convinced he needs a more combative footing against President Obama in order to appeal to white, working-class voters and to persuade them that he is the best answer to their economic frustrations…

Republican strategists said that many middle-class voters had proved reluctant to give up entirely on Mr. Obama, and that they still needed to be convinced that Mr. Romney would look out for their interests…

“They’re somewhat seduced by the thought, ‘If the guy had more time, maybe he’d be able to turn it around,’ ” said Mr. Law, whose group is spending tens of millions of dollars to change that…

The moves reflect a campaign infused with a sharper edge and overtones of class and race. On Friday, Mr. Romney said at a rally that no one had ever had to ask him about his birth certificate, and Mr. Ryan invoked his Catholicism and love of hunting. Democrats angrily said Mr. Romney’s remark associated him with the fringe “birther” camp seeking falsely to portray Mr. Obama as not American.