In the realm of politics, usually the Constitution of a country is prepared beforehand. And.when time comes to elect a'' government, or enact any legislation etc., every function is done and carried on according to that Constitution. Whatever conforms with it, is held valid and legal; whatever goes against that Constitution, is rejected as invalid and illegal. Since, according to the Sunni point of view, it was the duty of the Ummah to appoint a Caliph for themselves, it was necessary for Allah and His Prophet to give them a Constitution (with details of the procedure of election of such a Caliph). And if that was not done, then the Muslims themselves should have approved the Constitutional measures in advance before proceding to elect a Caliph.
But strangely enough it was not done. And now we find a unique 'unsettled Constitution' in which actions do not follow a Constitution (because there was none!), but the Constitution follows the events.
The best argument put forward by the Sunnis to support their claim is that the Muslims of first age considered it their duty to appoint a Caliph and that they regarded it so important that they neglected to attend the funeral of The Holy Prophet and went to Saqifa Bani Sa'ida to settle the question of Caliphate. From that event they conclude that the appointment of a Caliph was the duty of the Ummah.
But they fail to understand that it is the validity of that sery so-called 'election' which is challanged by the Shias.
The Shias claim that that event was illegal; the Sunnis claim that st was legal and correct. Now how can they put the very claim as their argunlent and proof?
To put the claim as the proof is just likc saying: "This action of mine is legal because I have done it"!
I do not know which court of justice will uphold such argument!