Adam Clarke

Reuben as the first-born had a right to a double portion of all that the father had; see Deu 21:17 The eminence or dignity mentioned here may refer to the priesthood; the power, to the regal government or kingdom - In this sense it has been understood by all the ancient Targumists. The Targum of Onkelos paraphrases it thus: "Thou shouldst have received three portions, the birthright, the priesthood, and the kingdom:" and to this the Targums of Jonathan ben Uzziel and Jerusalem add: "But because thou hast sinned, the birthright is given to Joseph, the kingdom to Judah, and the priesthood to Levi." That the birthright was given to the sons of Joseph we have the fullest proof from Ch1 5:1.