God's original plan was for both king and priest to be the same person, but every since the fall, mankind has been trying to keep the two separate. When God delivered the nation of Israel from slavery in Egypt and called them to be His own people, He told them this:
Now therefore, if you will indeed obey My voice and keep My covenant, then you shall be a special treasure to Me above all people; for all the Earth is Mine. 6) And you shall be to Me a kingdom of priests and a holy nation. These are the words which you shall speak to the children of Israel. (Exodus 19:5-6)
It was God's desire to bless all the nations and peoples of the Earth through Israel, as He had promised Abraham centuries earlier. That is why He called the Israelites a "Kingdom of priests"; they were to be His representatives before the rest of the world. Although the nation of Israel as a whole failed in this regard, God did not abandon His original plan and design. In the fullness of time, Jesus came, in the flesh a son of Israel, but in the Spirit, God's promised blessing to the world.
God has always wanted a priest with a crown. Now keep in mind, the problem with democracy, republic, monarchy, dictatorship, or any other system of human government is that they separate the offices of king and priest. Remember the devil is the deceiver and he has blinded the people from the truth of God's original plan. In a fallen world, it is probably a wise and necessary concession, because with sinful people the power of the state combined with the power of religion easily becomes overwhelmingly oppressive. Look at the state that this present world is in now. Separate offices of king and priest can serve as balances against each other.