Article 1, Section 3, Clauses 1 and 2

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.

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.

1.George Read to John Dickinson, 17 Jan. 1787
2.Credentials of Delaware's Delegates to the Federal Convention, 3 Feb. 1787
3.Records of the Federal Convention
4."John DeWitt," NO. 4, Fall 1787
5.James Wilson, State House Speech, 6 Oct. 1787
6.James Madison to Thomas Jefferson, 24 Oct. 1787
7.A Democratic Federalist, 26 Nov. 1787
8.Luther Martin, Maryland House of Delegates, 29 Nov. 1787
9.Luther Martin, Genuine Information, 1788
10.Debate in Massachusetts Ratifying Convention, 19 Jan. 1788
11.James Madison, Federalist, no. 62, 415--22, 27 Feb. 1788
12.James Madison, Federalist, no. 63, 422--31, 1 Mar. 1788
13.Brutus, no. 16, 10 Apr. 1788
14.Debate in New York Ratifying Convention, 24--25 June 1788
15.Debate in North Carolina Ratifying Convention, 25 July 1788
16.Charles Pinckney to James Madison, 28 Mar. 1789
17.Roger Sherman to John Adams, July 1789
18.St. George Tucker, Blackstone's Commentaries 1:App. 195--97, 215--25, 1803
19.Joseph Story, Commentaries on the Constitution 2:§§ 697, 699--710, 1833