The day after Standard & Poor?s took the unprecedented step of stripping the US government of its top credit rating, the ratings agency offered a full-throated defense of its decision, calling the bitter stand-off between President Obama and Congress over raising the debt ceiling a ?debacle.? It warned that further downgrades may lie ahead. Senior S&P officials insisted the ratings firm hadn?t overstepped its bounds by focusing on the political paralysis in Washington as much as fiscal policy in determining the new rating.