Mishnah Horayos 3:3:
וְאֵיזֶהוּ הַנָּשִׂיא, זֶה הַמֶּלֶךְ, שֶׁנֶּאֱמַר (ויקרא ד) וְעָשָׂה אַחַת מִכָּל מִצְוֹת ה' אֱלֹהָיו, נָשִׂיא שֶׁאֵין עַל גַּבָּיו אֶלָּא ה' אֱלֹהָיו
Who is meant by a ruler? A king; for it says, “Any of all the commandments of the Lord his God” (Leviticus 4:22), a ruler (king) who has none ...
This was also the opinion of Rav Avraham Yitzchak HaKohein Kook (1865-1935). I remember reading about this in Simcha Raz's biography on Rav Kook, titled "An Angel Among Men."
Can't find my copy of the book, but found the same idea attributed to Rav Kook in a Times Of Israel article: (among other results)
According to Rabbi Kook, the process of national ...