Digest of Biblical Civil Law

If the Lord's immediate designation of David through Samuel [1 Sam. 16] had been that which alone, without the election of the people, made David formally king of Israel, then there were two kings in Israel at one time. But David, after he himself was anointed by Samuel, many times calls Saul the Lord's anointed, and that by the inspiration of God's Spirit, as we and royalists do both agree. Two lawful supreme monarchs in one kingdom is as repugnant as two most highs or as two infinites. If David were king between his anointing by Samuel and his coronation by the vote of all Israel at Hebron [2 Sam. 2:3-4], then he failed to exercise his royal duty, as given by God, to execute justice and judgment, and defend religion. All David's suffering, upon David's part, must be unjust, for, as king, he should have cut off the murderer Saul, who killed the priests of the Lord; Saul, by this ground, must be a private murderer, and David the only lawful king. David, if he was formally king, deserted his calling in flying to the Philistines; for a king should not forsake his calling upon any hazard, even of his life, no more than a pilot should give over the helm in an extreme storm. Saul, David and Solomon (cf. 1 Ki. 1) remained private men until the election of the people. Therefore there flows something from the power of the people by which he who is no king now becomes a king formally, and by God's lawful call.
- Samuel Rutherford, Lex, Rex, Question 4 (1644), at http://www.constitution.org/sr/q04.txt.