The Federalist (1788)

2. It is equally unnecessary to dilate on the appointment of senators by the state legislatures. Among the various modes which might have been devised for constituting this branch of the government, that which has been proposed by the convention is probably the most congenial with the public opinion. It is recommended by the double advantage of favouring a select appointment, and of giving to the state governments such an agency in the formation of the federal government, as must secure the authority of the former, and may form a convenient link between the two systems.

3. The equality of representation in the senate is another point, which, being evidently the result of compromise between the opposite pretensions of the large and the small states, does not call for much discussion.