JACOB POSSESSED THE POSITION OF KING / PRIEST AND FIRSTBORN

Midrash Rabbah - Numbers 6:2

TAKE THE SUM OF, etc (Numbers 4:22).

… And so Jacob says to Reuben: Reuben, you are my firstborn... the excellency of dignity (se'eth), and the excellency of power--’oz’ (Genesis 49:3). By se'eth [lit. ‘lifting up’] he was alluding to the priesthood; for you read, And Aaron lifted up his hands toward the people, and blessed them (Leviticus 9:22) while by ’oz’ he alluded to the kingship; for it says, And He will give strength (‘oz’) unto His king (I Samuel 2:10). Had Reuben not disgraced himself by his conduct with Bilhah he would have been worthy of assuming the priesthood and the kingship seeing that he was the firstborn …