The passage of the Affordable Care Act would be his signature legislative achievement, but it propelled Republicans to a sweeping midterm election victory and control of the House. The fatalism Mr. Obama expressed privately captured the weighty consequences of one of Washington’s most high-wire legislative battles in modern times. A new set of oral histories released on Friday, on the eve of its 14th anniversary on Saturday, documents the behind-the-scenes struggle to transform the nation’s health care system.