St. Thomas on best form of government

[Geremia]

St. Thomas himself said in his Summa:

Unde optima ordinatio principum est in aliqua civitate vel regno, in qua unus praeficitur secundum virtutem qui omnibus praesit; et sub ipso sunt aliqui principantes secundum virtutem; et tamen talis principatus ad omnes pertinet, tum quia ex omnibus eligi possunt, tum quia etiam ab omnibus eliguntur. Talis enim est optima politia, bene commixta ex regno, inquantum unus praeest; et aristocratia, inquantum multi principantur secundum virtutem; et ex democratia, idest potestate populi, inquantum ex popularibus possunt eligi principes, et ad populum pertinet electio principum.

Accordingly, the best form of government is in a state or kingdom, where one is given the power to preside over all; while under him are others having governing powers: and yet a government of this kind is shared by all, both because all are eligible to govern, and because the rules are chosen by all. For this is the best form of polity, being partly kingdom, since there is one at the head of all; partly aristocracy, in so far as a number of persons are set in authority; partly democracy, i.e. government by the people, in so far as the rulers can be chosen from the people, and the people have the right to choose their rulers.

Pope Pius VI said monarchy is the best form of government in thesis ("praestantioris monorchici regiminis forma")
{allocution to the Consistory of June 17, 1793, Les Enseignements Pontificaux – La Paix Interieure de Nations, by the monks of Solesmes (Paris: Desclee & Cie), p. 8.}

U.S. presidents are definitely not kings. The U.S. is not "a state or kingdom, where one is given the power to preside over all". The U.S. is tricephalous (three-headed): congress, president, supreme court.

[tacf]

For sure a monarchy is best in the ideal. If you want to achieve a goal you have to intend the goal; each additional co-ruler is a potential source of disunity.
St Thomas De Regno:
"This question may be considered first from the viewpoint of the purpose of government. The aim of any ruler should be directed towards securing the welfare of that which he undertakes to rule. The duty of the pilot, for instance, is to preserve his ship amidst the perils of the sea. and to bring it unharmed to the port of safety. Now the welfare and safety of a multitude formed into a society lies in the preservation of its unity, which is called peace. If this is removed, the benefit of social life is lost and, moreover, the multitude in its disagreement becomes a burden to itself. The chief concern of the ruler of a multitude, therefore, is to procure the unity of peace. It is not even legitimate for him to deliberate whether he shall establish peace in the multitude subject to him, just as a physician does not deliberate whether he shall heal the sick man encharged to him, for no one should deliberate about an end which he is obliged to seek, but only about the means to attain that end. Wherefore the Apostle, having commended the unity of the faithful people, says: "Be ye careful to keep the unity of the spirit in the bond of peace." Thus, the more efficacious a government is in keeping the unity of peace, the more useful it will be. For we call that more useful which leads more directly to the end. Now it is manifest that what is itself one can more efficaciously bring about unity than several—just as the most efficacious cause of heat is that which is by its nature hot. Therefore the rule of one man is more useful than the rule of many.

Furthermore, it is evident that several persons could by no means preserve the stability of the community if they totally disagreed. For union is necessary among them if they are to rule at all: several men, for instance, could not pull a ship in one direction unless joined together in some fashion. Now several are said to be united according as they come closer to being one. So one man rules better than several who come near being one.

Again, whatever is in accord with nature is best, for in all things nature does what is best. Now, every natural governance is governance by one. In the multitude of bodily members there is one which is the principal mover, namely, the heart; and among the powers of the soul one power presides as chief, namely, the reason. Among bees there is one king bee' and in the whole universe there is One God, Maker and Ruler of all things. And there is a reason for this. Every multitude is derived from unity. Wherefore, if artificial things are an imitation of natural things' and a work of art is better according as it attains a closer likeness to what is in nature, it follows that it is best for a human multitude to be ruled by one person."

However, I look at America now and think that the division of powers is a useful good in the sense that less evil can be effected than if it were a tyranny. For example, the courts thus far striking down the abominations termed "vaccine mandates".

As St Thomas also says,
"Just as the government of a king is the best, so the government of a tyrant is the worst.

For democracy stands in contrary opposition to polity, since both are governments carried on by many persons, as is clear from what has already been said; while oligarchy is the opposite of aristocracy, since both are governments carried on by a few persons; and kingship is the opposite of tyranny since both are carried on by one person. Now, as has been shown above, monarchy is the best government. If, therefore, "it is the contrary of the best that is worst." it follows that tyranny is the worst kind of government.

Further, a united force is more efficacious in producing its effect than a force which is scattered or divided. Many persons together can pull a load which could not be pulled by each one taking his part separately and acting individually. Therefore, just as it is more useful for a force operating for a good to be more united, in order that it may work good more effectively, so a force operating for evil is more harmful when it is one than when it is divided. Now, the power of one who rules unjustly works to the detriment of the multitude, in that he diverts the common good of the multitude to his own benefit. Therefore, for the same reason that, in a just government, the government is better in proportion as the ruling power is one-thus monarchy is better than aristocracy, and aristocracy better than polity—so the contrary will be true of an unjust government, namely, that the ruling power will be more harmful in proportion as it is more unitary. Consequently, tyranny is more harmful than oligarchy; and oligarchy more harmful than democracy."