The President of the United States has powers granted by the Constitution. The Constitution further provides a separation of powers that limits the extent to which the President can enforce their will.
Filibusters also give a tremendous amount of power to individual Senators. Senators have used the filibuster, or the threat to filibuster in order to maximize their leverage with the President or other Senators.
The delegates to the convention included lawyers (thirty-four had studied law), soldiers (twenty-one fought in the Revolutionary War), farmers, educators, ministers, doctors, bankers and merchants.