St. Thomas Aquinas writes regarding the ideal,
“One of the principal things to be considered,” Thomas wrote, “with regard to the good establishment of princes [rulers] is that all should have some part in the government; for in this way peace is preserved amongst the people, and all are pleased with such a disposition of things and maintain it.
“The next thing to be considered is the form of government, of which there are principally two kinds:
“A Kingdom, in which one rules, and an Aristocracy, in which a few exercise the authority.
“The best form is that in which one rules over all, and under him there are others having authority, but the government pertains to all, because those who exercise authority can be chosen from all and are chosen by all.
“Hence the best government is a mixture of a Kingdom, of Aristocracy and of Democracy, i.e., of the power of the people, inasmuch as the rulers can be chosen from the people, and the election of the rulers belongs to the people.” — 1a 2ae, question 105, article first.
“There is a vast amount of good republicanism and of sound democracy in these words. First, by the kind or monarch St. Thomas means nothing more than some one is to represent the governing authority. Secondly, the aristocracy means those who exercise a salutary restraint on the power of the head of the government; because if there were no restraint the power of the king, says St. Thomas (ad 2um), would easily degenerate into a tyranny. Congressmen and senators [or the equivalents], for instance, would supply the demand for an aristocracy.
“Lastly, St. Thomas says that neither kingdom nor an aristocracy will form a stable government unless the element of democracy is introduced…” Comment by stmaryvalleybloom.org
Add to this of course the principles of Magna Carta, 1215
— Rerum Novarum. On Capital and Labor, Pope Leo XIII
Catholic Critique 