AHC: Reunite the United States and the British Empire with POD after 1848

Looking at a map of shipping lanes in the 19th century, you'd hardly be able to tell that the US and British Empire were different countries, despite protectionist policies in the United States in the 1800s. How could the economic link between Britain and the United States be made into a political unification? The POD must be after 1848, when the US has reached essentially the full size of the Contiguous 48 (save the Gadsden Purchase) and the northern states have developed to a recognizable extent.

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