Final answer:
Many Muslims believed Ali was the rightful caliph due to his direct family ties to the Prophet Muhammad and claims that Muhammad had chosen him as his successor, especially held by the Shia sect of Islam.
Step-by-step explanation:
Reasons for Ali's Support as the Rightful Caliph
Many Muslims believed that Ali was the rightful caliph and heir of Muhammad because of Ali's close family ties to the Prophet, being Muhammad's cousin and son-in-law, having married the Prophet's daughter, Fatima. Moreover, Ali was seen as having been chosen by Muhammad in various historical accounts, although this was not universally accepted across all Muslim factions. The Shia Muslims particularly hold the belief that there was an implied succession for Ali from the Prophet. Ali's followers were known as the Shi'a, and this group had a fundamental belief in the legitimacy of Ali's claim to the caliphate over others. It was these beliefs and Ali's ties to Muhammad that led to his supporters, and later the Shia sect, regarding him as the rightful successor.