Article 4, Section 4

The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.

1.Vermont Constitution of 1777, CH. 1, ARTS. 4--8
2.James Madison, Vices of the Political System of the United States, Apr. 1787
3.Records of the Federal Convention
4.Tench Coxe, An Examination of the Constitution of the United States of America, Fall 1787
5.William Symmes to Capt. Peter Osgood, Jr., 15 Nov. 1787
6.Alexander Hamilton, Federalist, no. 21, 130--32, 12 Dec. 1787
7.James Madison, Federalist, no. 43, 291--95, 23 Jan. 1788
8.James Madison, Virginia Ratifying Convention, 6 June 1788
9.Debate in Virginia Ratifying Convention, 14 June 1788
10.James Wilson, Of Man in Confederation, Lectures on Law, 1791
11.St. George Tucker, Blackstone's Commentaries 1:App. 366--67, 1803
12.Johnson v. Duncan
13.William Rawle, A View of the Constitution of the United States 295--304, 305--7 1829 (2d ed.)
14.Joseph Story, Commentaries on the Constitution 3:§§ 1808, 1819, 1833