Madison was afraid that states will admit such large numbers of "alien residents" to the point that they become the majority of the people in the state. These people, Madison suggests, would become "an unhappy species of population" living below the average means ("are sunk below the level of men") who ultimately resort to riots and violence to elevate their cause with the purpose of rallying support.
This latter prediction is coming true right now with the riots of 2020 and the open borders allowing millions of aliens to enter the country uncontrolled and unaccounted.
From Federalist #43 (James Madison):
6. "To guarantee to every State in the Union a republican form of government; to 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....A protection against invasion is due from every society to the parts composing it. The latitude of the expression here used seems to secure each State, not only against foreign hostility, but against ambitious or vindictive enterprises of its more powerful neighbors. The history, both of ancient and modern confederacies, proves that the weaker members of the union ought not to be insensible to the policy of this article. Protection against domestic violence is added with equal propriety. It has been remarked, that even among the Swiss cantons, which, properly speaking, are not under one government, provision is made for this object; and the history of that league informs us that mutual aid is frequently claimed and afforded; and as well by the most democratic, as the other cantons. A recent and well-known event among ourselves has warned us to be prepared for emergencies of a like nature. At first view, it might seem not to square with the republican theory, to suppose, either that a majority have not the right, or that a minority will have the force, to subvert a government; and consequently, that the federal interposition can never be required, but when it would be improper. But theoretic reasoning, in this as in most other cases, must be qualified by the lessons of practice. Why may not illicit combinations, for purposes of violence, be formed as well by a majority of a State, especially a small State as by a majority of a county, or a district of the same State; and if the authority of the State ought, in the latter case, to protect the local magistracy, ought not the federal authority, in the former, to support the State authority? Besides, there are certain parts of the State constitutions which are so interwoven with the federal Constitution, that a violent blow cannot be given to the one without communicating the wound to the other. Insurrections in a State will rarely induce a federal interposition, unless the number concerned in them bear some proportion to the friends of government. It will be much better that the violence in such cases should be repressed by the superintending power, than that the majority should be left to maintain their cause by a bloody and obstinate contest. The existence of a right to interpose, will generally prevent the necessity of exerting it.
...May not the minor party possess such a superiority of pecuniary resources, of military talents and experience, or of secret succors from foreign powers, as will render it superior also in an appeal to the sword? May not a more compact and advantageous position turn the scale on the same side, against a superior number so situated as to be less capable of a prompt and collected exertion of its strength? Nothing can be more chimerical than to imagine that in a trial of actual force, victory may be calculated by the rules which prevail in a census of the inhabitants, or which determine the event of an election! May it not happen, in fine, that the minority of CITIZENS may become a majority of PERSONS, by the accession of alien residents, of a casual concourse of adventurers, or of those whom the constitution of the State has not admitted to the rights of suffrage? I take no notice of an unhappy species of population abounding in some of the States, who, during the calm of regular government, are sunk below the level of men; but who, in the tempestuous scenes of civil violence, may emerge into the human character, and give a superiority of strength to any party with which they may associate themselves. In cases where it may be doubtful on which side justice lies, what better umpires could be desired by two violent factions, flying to arms, and tearing a State to pieces, than the representatives of confederate States, not heated by the local flame? To the impartiality of judges, they would unite the affection of friends. Happy would it be if such a remedy for its infirmities could be enjoyed by all free governments; if a project equally effectual could be established for the universal peace of mankind! Should it be asked, what is to be the redress for an insurrection pervading all the States, and comprising a superiority of the entire force, though not a constitutional right? the answer must be, that such a case, as it would be without the compass of human remedies, so it is fortunately not within the compass of human probability; and that it is a sufficient recommendation of the federal Constitution, that it diminishes the risk of a calamity for which no possible constitution can provide a cure. Among the advantages of a confederate republic enumerated by Montesquieu, an important one is, "that should a popular insurrection happen in one of the States, the others are able to quell it. Should abuses creep into one part, they are reformed by those that remain sound. ''
-PJ
BTTT!