Thank you for the question on positional authority, “Who had more power in Biblical Israel, the king or the prophet?”
The king of Israel had civil authority over the people of Israel while a prophet had spiritual/religious authority over the people of Israel. Between the civil and religious authority, the king would have greater power in the sense of physical might through the loyalty of the soldiers while the prophet had no physical army with him to oppose a rebellious or idolatrous king.
In II Kings 1, the biblical narrative states a power struggle between King Ahaziah and Elijah. King Ahaziah became ill and wanted to consult the god of Ekron about his recovery. Elijah was directed to the messengers of Ahaziah to give him the message that he will not recover.
The text states this, “The king said, “That was Elijah the Tishbite.” Then he sent to Elijah a captain with his company of fifty men. The captain went up to Elijah, who was sitting on the top of a hill, and said to him, “Man of God, the king says, ‘Come down!’” 10 Elijah answered the captain, “If I am a man of God, may fire come down from heaven and consume you and your fifty men!” Then fire fell from heaven and consumed the captain and his men.” (NIV).
For one’s consideration, spiritual authority is greater than civil authority under a theocracy, not a democracy. In a democracy/republic, the civil government is seen as the ultimate authority over a person’s position, possessions and even physical life, while in a theocracy the spiritual leader is the ultimate leader over the person’s spiritual life which may impact one’s position, possessions and physical life.
SUMMARY: In a theocracy, the king is under the rule of God. Thus a prophet who is sent by God with God’s message should have greater authority over a king.
For further insight and discussion:
https://www.quora.com/Who-had-more-power-in-Biblical-Israel-the-king-or-the-prophet