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.
The argument could be made that a tyrannical government might purposely admit such numbers of "alien residents" so they can incite them to riot against the regular order in 2024 the way they got ANTIFA and BLM to riot in the cities in 2020.
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."
The last bolded point is why Biden has been spreading the illegal aliens across the entire country. This way, there are no more states that "remain sound" that can "quell... a popular insurrection" in neighboring states.
-PJ
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Were it admitted however that the federal government may feel an equal disposition with the state governments to extend its power beyond the due limits, the latter would still have the advantage in the means of defeating such encroachments. If an act of a particular state, though unfriendly to the national government, be generally popular in that state, and should not too grossly violate the oaths of the state officers, it is executed immediately, and of course, by means on the spot, and depending on the state alone. The opposition of the federal government, or the interposition of federal officers, would but inflame the zeal of all parties on the side of the state, and the evil could not be prevented or repaired, if at all, without the employment of means which must always be resorted to with reluctance and difficulty. On the other hand, should an unwarrantable measure of the federal government be unpopular in particular states, which would seldom fail to be the case, or even a warrantable measure be so, which may sometimes be the case, the means of opposition to it are powerful and at hand. The disquietude of the people, their repugnance and perhaps refusal to co-operate with the officers of the union, the frowns of the executive magistracy of the state, the embarrassments created by legislative devices, which would often be added on such occasions, would oppose in any state difficulties not to be despised; would form in a large state very serious impediments, and where the sentiments of several adjoining states happened to be in unison, would present obstructions which the federal government would hardly be willing to encounter.
Biden is the insurrectionist!