If the US loses the Cold War, say due to Khrushev's economic reform attempts in 1960s being more successful, and 1990 Russia being essentially similar economically to present-day China (in terms of efficiency and economic growth), then the US could fall into the "third world status", the only problem would be their possession of nukes.
Say, Reagan's attempted military buildup is easily matched by the Soviets, and by mid-1990s the general economic discontent in the US is at the boiling point, resulting in riots and street battles. Few foreign interventionist adventures that went sour do not endear the people to the government, and the perception that the large corporations, primarily oil companies and military/industrial complex, get richer while your average Joe barely gets by will make secessionist movements gain strength. Then, expect a USSR-style scenario, except on the other side of Atlantic, with fractured US practically descending into Civil War (as the federal government would not let any of the states break away without a fight).
Eventually the other world powers step in to broker an agreement, resulting in removal of the US nuclear weapons in return for economic and military aid, and Russian, British, Japanese, French, German, and Brazilian peacekeeping troops garrisoned among the former US. In 2005, the US economy is in ruins, its government barely able to maintain shaky control over various states, secessionist movements are still going strong, and without the nuclear weapons it is generally considered a world pariah.