Final answer:
The U.S. founders focused on Montesquieu's idea of separation of powers and Rousseau's concept of popular sovereignty as the basis for the governmental foundations.
Step-by-step explanation:
The motive for the U.S. founders to focus on Montesquieu's and Rousseau's ideas as a basis for the governmental foundations was due to the influence of their political theories. Montesquieu's idea of separating powers into three branches (legislative, executive, and judicial) helped shape the U.S. system of checks and balances, while Rousseau's concept of popular sovereignty influenced the idea of government deriving its power from the consent of the governed.
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