Final answer:
Severe economic problems are the most common reason for one party losing control of the national government.
Step-by-step explanation:
In American history, the most common reason for one party losing control of the national government is when the country suffers from severe economic problems. Economic issues can greatly influence voter sentiment and lead to a change in political power. For example, during the Great Depression in the 1930s, the Democratic Party gained control due to public dissatisfaction with the Republican Party's handling of the economic crisis.
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