Although majority of Sunni sources indicate that Imam Ali (A.S) finally pledged allegiance to Abu Bakr, and he became one of the main supporters of the system, the reality is otherwise, as according to many companions and later on historians, Imam Ali (A.S) never pledged allegiance to them and refused to present any sort of legitimacy.
According to unanimous narrations by both groups (Shia & Sunni), after the attack on Imam Ali’s (A.S) House, the Imam (A.S) was tied and dragged to the mosque, as he was asked forcefully to pledge allegiance to Abu Bakr. Imam Ali (A.S) refused and he was threatened to be killed, but since they knew killing the Imam (A.S) is literally impossible and would cause more harm to their purpose, they decided to use another method. According to historians, Omar and others pulled the Imam (A.S) toward Abu Bakr and Abu Bakr came toward the Imam (A.S), Omar pulled the hand of Imam Ali (A.S) and placed Abu Bakr’s hand on top it and shouted: “O’ everyone, Ali pledged to the new Caliph.”
Therefore, even during these tense and sensitive moments, Imam Ali (A.S) refused to give any kind legitimacy to the new system, as he refused to pledge his allegiance to Abu Bakr while saving the stability of his society.