הלכה על קידושין 152:14

He prohibited us - that we not appoint as a king upon ourselves, a man who is not of the seed of Israel, even if he be a righteous convert. And that is His saying, "you may not place over yourself a foreigner who is not among your brothers" (Deuteronomy 17:15). And the language of the Sifrei (Sifrei Devarim 157:9) is, "'You may not place over yourself a foreigner' - that is a negative commandment." And likewise with the rest of the appointments: It is not permissible that we appoint any of the appointments over us - whether it is a Torah position or a governmental position - a man that is from the congregation of converts, until his mother be an Israelite. About His saying, "Surely appoint over yourselves a king [...], a king from among your brothers shall you appoint over yourselves," they said (Kiddushin 76b), "Any appointments that you appoint shall only be from among your brothers." Of course, concerning the monarchy specifically, you have already known from the books of prophecy that David - and, likewise, his seed after him - have already acquired it: For all generations, there is no king for one who believes in the Torah of Moshe - the master of all prophets - besides one from the seed of Shlomo, exclusively. And anyone who is not from that glorious seed is called, an outsider, regarding the monarchy - just like anyone who is from any other seed besides the seed of Aharon is called, an outsider, regarding the [Temple] service. And this is clear - there is no doubt about it. And the regulations of this commandment have already been explained in [various] places in Yevamot, Sotah and Sanhedrin. (See Parashat Shoftim; Mishneh Torah, Kings and Wars 1.)

To not establish a foreign king over us: That we were prevented from establishing a man that is not from the seed of Israel - and even if he is a righteous convert - [as] king upon us. And about this is it stated (Deuteronomy 17:15), "You may not place upon yourself a foreign man that is not your brother." And they, may their memory be blessed, said in Sifrei Devarim 157, "'You may not place upon yourself a foreign man' - this is a negative commandment." And likewise, it is not fitting to appoint upon us for anything - not a Torah appointment and not a state appointment - a man that is from the 'congregation of converts,' until his mother be from Israel, from that which is written, "You shall surely place, etc."; and they, may their memory be blessed, made a precise inference (Kiddushin 76b), "Any placing that you do shall only be from among your brothers."