As others have said, the US in the OP scenario would still very much be a superpower. Losing Oregon in the 1860s is a joke, a century of confederate resistance is a fantasy, Alaska an irrelevance. If the Soviet Union was a superpower, so would be this USA, which would be stronger.
Obviously having a devastating revolution isn't enough to remove Russia. It took two revolutions, two utter disasters in world wars, and no little misrule to do it in OTL, after all.
British-led Imperial Federation, China, France-dominated Europe, Germany-dominated Europe, a federalized Spanish America, and India are the other possible contenders. India is safely out of the running by about 1750 anyway, unless its wagging the dog of the Imperial Federation. By 1815 France and New Spain both were out of the question as well. Britain wasn't a superpower, but was about as near as a great power could get. It could though have managed as the nucleus of a great power with the right POD spurring Imperial Federation before 1880 or so. Germany went out the window for all intents and purposes in 1933 - not that no victory was possible - just a victory that would leave them a genuine superpower. China could easily have gotten there as late as 1925, and still had an outside chance as late as the 1950s of reaching that status by today.
So the trick is that we have to, as OTL, break Russia and China, while simultaneously eliminating the USA, and we have to do all that without accidentally opening an opportunity for one of the potential contenders of OTL.