Being a continental power and an oceanic power is the reason the United States is a superpower (and indeed has been since the late 1800's). Being a continental power gives it both unrivalled access to resources and the ability to be essentially self sufficient in virtually every regard (fuel, mineral resources, farm produce, forestry etc. are generally sufficient to supply all of America's needs and even surplus to export), but the vast size of America provided lots of opportunities and need to develop new technologies, and the size of the internal market gave American industry the ability to grow a superpower sized economy. Using the fairly simple metric of steel production to gage the size of the economy, the US surpassed European powers starting in the mid 1800's, and was the dominant power by the turn of the 20th century. Even if European economies hadn't self destructed in a series of World Wars, America would still be by far the largest economic power today due to the advantages of being a continental power.
Real continental powers have one huge disadvantage; they must devote an inordinate amount of time and resources to securing their land borders (Russia and Germany have wrestled with that issue for ages). The US is a Continental power bounded by two fairly weak polities, but also an Oceanic power, which means that potential enemies need to arrive by sea, but also the US has the unique ability to project trade and military power by sea into both of the world's major oceans. India, for example, can only project power freely into the Indian Ocean, access to the Pacific is blocked by narrow passageways through multiple island chains and access to the Atlantic is through rounding Africa. China needs to transit the "First Island Chain" in order to project power into the Pacific, and faces the same problem that India has in reverse when considering the Indian ocean.
So if America's access to the West coast had been blocked by Spain or the British Empire (there was a contest for the Oregon region between Britain and the US, for example), then the US would still be a powerful nation, but far less dominant. American military power would be preferentially focused on defending the western borders (or being tailored to defeat the Spanish and British to take the West Coast). I suspect that if the US had been decisively blocked in the west, then the purchase of Alaska from Russia would never have taken place (there would have been fewer resources to do so, and less perceived need as well), and of course Hawaii would be either an independent Kingdom or a possession of the British Empire.
Without the ability to project power into the Pacific, the history of Japan would be different (no American "Black Fleet" to open Japan to the modern world), and Chinese history would also be different, since American missionaries opened American eyes to the potential of China in the late 1800's, and American policy towards China until the Communist takeover was based on the "Open Door" trade policy and the "Long Watch" of American military power deployed to China to ensure that free trade could take place. How exactly this would play out is hard to determine, but I can imagine more "Boxer rebellion" type events against the other Western Empires and more discord and disorder within China when the Qing dynasty finally fell.
If you want a contrafactual to make this happen without the Spanish or British Empires, you might look to the Civil War. If the Copperheads had won the election and ended the Union war effort, the American nation would probably have split into 3 parts; a diminished Union, a Confederacy and a new nation west of the 100th meridian which was anti slavery but not interested in being subservient to the more populous Union.