(Genesis 49:3, continued:) "And excelling in power" — the kingship had been fitting for you, for there is no "power" except kingship, as it is said (1 Samuel 2:10): "And He shall give power to His king." And one said it was said in praise. "Reuben, you are my firstborn" — you are fitting for the birthright, even though you profaned it, for he was zealous with the zeal of his mother. Why? All the days that Rachel was alive, the bed of Jacob was beside her. When she died, Jacob took his bed and placed it beside Bilhah. What did Reuben do? He took up the zeal of his mother. He made himself as though he were sleeping upon that bed, so that Jacob would take his bed beside Leah. Therefore he is praising him: "Reuben, you are my firstborn, my might and the first of my strength" — you were the first drop that came forth from me.
Why Jacob Still Praised Reuben as His Firstborn
Midrash Tanchuma Buber, Vayechi 11:5
ויתר עז שלך היתה המלוכה ראויה, ואין עז אלא מלכות, שנאמר ויתן עז למלכו (ש"א ב י). וחד אמר לשבח, ראובן בכורי אתה, נאה אתה לבכורה, אע"פ שחללת בקנאתו של אמו קינא, למה כל ימים שהיתה רחל קיימת היתה מטתו של יעקב אצלה, משמתה נטל יעקב מטתו ונתנה אצל בלהה, מה עשה ראובן נטל קנאתו של אמו, עשה עצמו כאילו ישן באותה מטה, כדי שיטול יעקב מטתו אצל לאה, לפיכך הוא מ קלסו, ראובן בכורי אתה, כחי וראשית אוני, אתה הייתה טפה ראשונה שיצתה ממני.