Of course there isn't one; the notion of the "rightful king" in Middle-Earth does not have a real world counterpart.

Tolkien might have believed it did, since he was a Catholic and might have believed in some version of the divine right of kings that the church supported for many centuries. But even then, the power the "rightful king" has in Middle-Earth is very limited. There is no hint that Aragorn, once he becomes King, micromanages everything in Gondor or makes rules by royal decree about everything, or even any very great number of things. The only actual official acts of his that are described are making peace with the Haradrim and the Easterlings, giving Sauron's freed servants the lands about Lake Nurnen, and pronouncing judgments of particular cases, of which Beregond's is the last. He certainly doesn't seem to be dictating what everyone in Gondor should do in their daily lives. Nor is there any hint that previous Kings did any such thing.

And even Tolkien's real world attitudes weren't necessarily monarchist. In a letter to his son, he wrote:

"The most improper job of any man, even saints (who at any rate were at least unwilling to take it on), is bossing other men. Not one in a million is fit for it, and least of all those who seek the opportunity..."

If this espouses any kind of political view, it's libertarianism.