The continuing obligation of the United States with reference to the existence of the states is twofold: The constitution provides (Art. IV, § 4), for (1) a republican form of government in each state, and (2) the protection of such government against being overthrown by invasion of a foreign power or by domestic violence. It is evidently assumed in the guarantee that the forms of government existing in the different states at the time of the adoption of the federal constitution were republican. The characteristic feature of such a form of government is that those exercising authority do so in a representative capacity; it cannot be a monarchy on the one hand nor a pure democracy on the other. No doubt a republican form of government, as described in the federal constitution, involves the exercise of the powers of government by different departments, and a selection of the members of, at least, the lower branch of the legislature by popular vote, but by popular vote it is not necessarily meant that all the adults or even all the adult males shall be entitled to exercise the electoral franchise, but only that officers be in some form selected by a body of electors substantially representing the people. As will be pointed out in the chapter relating to citizenship and political privileges (see below, § 200) the right to vote is to be regarded only as a privilege conferred in accordance with the public interest.

While the constitution provides that the United States "shall" guarantee a republican form of government to the states, it is to be understood that the exercise of this power is discretionary. There has been, as yet, but little discussion as to the nature and extent of this power, for there has been little occasion for its exercise. It may be suggested, however, that republican government might cease to exist in a state (1) through invasion by a foreign power and an attempt to set up some other form of government by its authority; (2) or by a revolutionary attempt of the people themselves to overthrow the existing republican form of government and to substitute some other form in its place; (3) or by an attempt to destroy republican government by amendment of state constitutions. In any of these instances the new government would be illegal and unauthorized, and a republican form of government having ceased to exist there would be no state government and Congress would have occasion to provide for the establishment of such a government (see below, § 181).