I am inclined to agree with Janprimus. Every country thinks itself exceptional, different and unique. With imo is true to an extent, for the US too, don't think you can change that without changing the very nation and how it develops. The US is simply likely to develop differently politically from most European countries due to how much easily conquerable land it has to its west. Compared to Europe which was pretty filled up, and the competition for land was much stronger.
When it comes to America seeming exceptional during the 1940-2000 I would also argue that it has more to do with the European countries being devastated by the world wars, which politically, geopolitically and socially are still felt until today. With large parts of Europe still recovering from communist rule.