President Obama on Thursday night accepted the Democratic nomination for a second term, making a forceful argument that he had rescued the economy from disaster and ushered in a recovery that would be imperiled by a return to Republican stewardship. Defining his fight for re-election as a bald "choice between two fundamentally different visions for the future," Mr. Obama conceded the country's continuing difficulties while pleading for more time to solve them. He laid out a long-term blueprint for revival in an era obsessed with short-term expectations.