The discussion here concerns two matters:

The first is that Abū Bakr was the one entitled to the imamate, and that their pledge of allegiance (1) to him was something beloved to Allah and His Messenger. This is established by the revealed texts and consensus.

The second concerns the point at which he became imam: this occurred through the pledge of allegiance given to him by those who possessed effective power. Likewise, when Abū Bakr designated Umar as his successor, Umar became imam only when they pledged allegiance to him and obeyed him. Had they not implemented Abū Bakr’s designation and not pledged allegiance to Umar, he would not have become imam, regardless of whether that was permissible or impermissible.

Lawfulness and prohibition pertain to actions. Authority and rule themselves, however, consist in power that has actually been attained. Such power may be attained in a manner beloved to Allah and His Messenger, as with the rule of the Rightly Guided Caliphs; or it may be attained in a manner involving disobedience, as with the rule of tyrants.

Had Umar and a group with him pledged allegiance to Abū Bakr while the rest of the Companions withheld their allegiance, Abū Bakr would not thereby have become imam. He became imam only through the pledge of allegiance of the majority of the Companions, who possessed effective power and strength. Hence, Sa'd ibn Ubādah’s withholding of allegiance caused no harm, because that [did not] (2) undermine the intended purpose of authority. That purpose is the attainment of the power and rule through which the interests of the imamate are secured (3), and this had been achieved through the majority’s agreement to it.