Though they ought to have deferred their political rally...
Though they ought to have deferred their political rally until after the burial of the body of their master, at the moment it appeared to them that they had to act immediately or else it would be too late. As noted before, the Ansar had given sanctuary to Islam at a time when its situation was most forlorn. For the sake of Islam, they had made all Arabs their enemies. For the sake of Islam, they had pitted themselves against all Arabia.
In every battle of Islam, they had acquitted themselves most honorably. Many of their young men were killed in these battles. (In the battle of Uhud 75 Muslims were killed; out of them four were Muhajireen, and the rest were all Ansars). They demonstrated their devotion to Islam and their loyalty to the Prophet at every juncture. The Ansar knew that caliphate was Ali's right but they also knew about the “resolution” of the “Arabs” to keep caliphate out of the house of the Prophet.
Their interpretation of this “resolution” was that the Muhajireen would not let Ali reach the throne of caliphate. But if not Ali, then who else would be Muhammad's successor? The only obvious answer to this question was: some other Muhajir. But any Muhajir other than Ali was not acceptable to them – to the Ansar. They, therefore, decided to put forward their own candidate for the leadership of the umma .
After all it was their support, they argued, and not the support of the Muhajireen, that had made Islam viable. The anxiety of the Ansar is perfectly understandable. To them, the prospect of the government of Medina falling into the hands of the Umayyads, the traditional enemies of God and His Messenger, who had now become Muslim, was extremely frightful. They (the Ansar) had killed many of them in the battles of Islam.
If the government of Medina which was consolidated with their (the Ansars') support, was ever captured by the children of those pagans whom they (the Ansar) had killed, how would they treat them (the Ansar), was the unspoken question in the heart of every Ansari. Events proved that their fears were not generated by any hallucination. The Umayyads had fought bitterly against Islam and its Prophet.
When the latter captured Makkah, they “accepted” Islam because there was little else they could do then. As noted before, the Prophet never gave them any positions of authority even though he gave them a generous share out of the spoils of the battle of Hunayn.