The key character in question is Ja'far al-Sadiq. After replacing the Umayyads:

From their base in Kufa, the Abbasids sent missionaries to Khurasan, where they proclaimed the imminent coming of 'the preferred one from the house of Muhammad." The phrase was a brilliant piece of propaganda aimed at attracting as wide a range of Shi'i sympathizers as possible to Abbasid cause while simultaneously obscuring the identity of the Abbasid leadership.

Enter Ja'far al-Sadiq:

The Abbasid charade could not last. Once in power, Shi'i enthusiasm for the descendants of Ali threatened the legitimacy of of the Abbasids. Consequently the new rulers dispensed with the ficiton of Shi'i sympathy, and brutally repressed their erstwhile allies. They had their main Shi'i supporters murdered, including the leading propagandist of the movement, Abu Muslim. About seventy years later, Caliph Ma'mun (r. 813-33) made a bid to recapture Shi'i sympathies and return the Abbasids to their roots by naming the grandson of Ja'far al-Sadiq, the imam, Ali al-Rida, as his political successor. Ali al-Rida died before the plan could be tested, however, and the Abbasids had to continue to derive their political legitimacy from other sources.

After the rise of the Abbasids the Shi'i political ideal remained very much alive, in both quietist and militant forms. Ja'far al-Sadiq, who had witnessed the Abbasid rise to power, preached a politically quiescent Shi'ism, and during his lifetime the majority of Shi'i Muslims seem to have been united behind his imamate. Ja'far's teachings on the imamate were perfectly suited to help Shi'i Muslims make sense of their situation after their betrayal by the Abbasids. Acknowledgement of the true imam was a religious obligation, but it was not necessary to get oneself killed trying to get him into power.

This was a philosophy that advocated survival over unrestrained idealism. Anyway, the Abbasid betrayal described above marked the initial split in empire, at least in terms of ideology. This was further compounded after Ja'far's death when the Shi'i split into the Twelvers and the Isma'ilis.

Taken from Daniel Brown's A New Introduction to Islam, Chapter 9 The Caliphate.