Prince Yamashiro no Oe had a closer blood relation with the Soga clan (Prince Shotoku was biologically related to the Soga clan, and Prince Yamashiro no Oe's mother was a younger sister of Emishi), however, Emishi recommended Prince Tamura for the next Imperial Throne since he disliked Jogu Oke (Prince Shotoku's family line) to gain more power by choosing the competent prince Yamashiro no Oe for the next emperor.
血統的には山背大兄王の方が蘇我氏に近いが（聖徳太子は蘇我氏の血縁であり、山背大兄王の母は蝦夷の妹である）、有能な山背大兄王が皇位につき上宮王家（聖徳太子の家系）が勢力を持つことを嫌った蝦夷は田村皇子を次期皇位に推した。