It will save much time if I explain at the outset the basic cause of the differences concerning the nature and character of the Imamat and Caliphate.
What is the primary characteristic of Imamat? Is an Imam, first and foremost, the ruler of a Kingdom? Or is he, first and foremost, the Representative of Allah and the Vicegerent of the Prophet?
As Imamat and Caliphate is acceptedly the successorship of the Prophet, the above questions cannot be answered till we first decide what is the basic characteristic of a prophet. We must decide whether the prophet is, first and foremost, the ruler of a Kingdom, or is he, first and foremost, the Reprensentative of Allah.
We find in the history of Islam that there was a group which looked upon the Mission of the Holy Prophet as an attempt to establish a Kingdom. Those were the people who were waiting to see the outcome of the war between Muslims and Quraish. These were the Arabs who used to say that Mohammad(saww) be left to his people. If he would emerge victorious over them he is undoubtedly a true prophet. So, according to them, victory was the criterion of truth! If Mohammed(saww) would have been defeated he would have been treated as a liar!
The view that his sacred Mission was nothing but a worldly affairs was repeatedly announced by Abu Sufyan and his clan.
At the time of the Fall of Mecca, Abu Sufyan said to Abbas, "Abbas! Verily your nephew has acquired quite a Kingdom!" Abbas said, "Woe unto thee! This is not Kingship, it is the Prophethood."
Here you find the two opposite views in clear contrast. Abu Sufyan never changed his views. When Uthman became Caliph, Abu Sufyan came to him and advised, "O children of Umayya! Now that this Kingdom has come to you, play with it as the children play with a ball and pass it from one to another of your clan. This Kingdom is a reality; and we do not know whether there is any paradise and hell or not."
Then he went to Uhud and kicked at the grave of Hamza, and said, "O Abu Ya'ala! See that the Kingdom for which you were fighting us has at last come to us."
The same views were inherited by Yazid who said: "Banu Hashim staged a play to obtain Kingdom; actually, there was neither any news from God nor any revelation."
If that is the view held by any Muslim, then he is bound to equalize Imamat with Rulership. According to such thinking the primary function of the Prophet was Kingship, and, therefore, anybody holding the rein of power in his hand is the rightful successor of the Holy Prophet.
But the trouble is that more than 99 per cent of the prophets had no political power; and most of them were persecuted and apparently helpless victims of the political powers of their times. Their glory was not of crown and throne, but of martyrdom and sufferings. If primary characteristic of prophethood is the political power and rulership, then, perhaps not even 50 (out of 124,000) prophets would retain their Divine title of 'Nabi.'
Thus it is crystal-clear that the main characteristic of the Holy Prophet was not that he had political power but that he was the Representative of Allah. And that Representation was not bestowed upon him by his people; but by Allah Himself.
Likewise, his successor's Chief Characteristic can not be the political power; but the fact that he was the Representative of Allah. And that Representation can never be bestowed upon any one by his people; it must come from Allah Himself.
In short, if the Tmam is to represent Allah, he must be appointed by Allah.