Constitution of the United States – Article I sec 3

Section 3 deals with the composition of the Senate:

Section 3

(clause 1)

The Senate of the United States shall be composed of two Senators from each State, chosen by the Legislature thereof for six Years; and each Senator shall have one Vote.

Where the House represents each State according to its population, the Senate represents the States on an equal basis. This keeps the more populous states from drowning the voice of the less populous states.

Senators are also elected for a longer term than Representatives. This allows for the greater accumulation of experience among Senators. More expertise is asked and expected of them.

With equal representation between states, and being chosen by the state Legislature, Senators are apparently intended to be the professionals of Congress and represent their states at the governmental level rather than directly represent the citizens.

(clause 2)

Immediately after they shall be assembled in Consequence of the first Election, they shall be divided as equally as may be into three Classes. The Seats of the Senators of the first Class shall be vacated at the Expiration of the second Year, of the second Class at the Expiration of the fourth Year, and of the third Class at the Expiration of the sixth Year, so that one third may be chosen every second Year; and if Vacancies happen by Resignation, or otherwise, during the Recess of the Legislature of any State, the Executive thereof may make temporary Appointments until the next Meeting of the Legislature, which shall then fill such Vacancies.

Senators are divided into three classes, which are elected for overlapping terms. This allows for greater stability in the Senate than might be realized in the House.