In 1996, Thomas Friedman, the New York Times columnist, remarked on “The NewsHour With Jim Lehrer” that there were two superpowers in the world — the United States and Moody’s bond-rating service — and it was sometimes unclear which was more powerful. Moody’s was then a private company that rated corporate bonds, but it was, already, spreading its wings into the exotic business of rating securities backed by pools of residential mortgages.