Zaydi Shiʿism was initially formed during the 2nd/8th century by the merger of two currents in Kufan Shiʿism; the two doctrinal currents with their Shiʿi adherents were designated in heresiographical literature as the Batriyya and the Jarudiyya, also referred to as the ‘weak’ and the ‘strong’ Zaydis, respectively.
The main disagreement regarding the concept of imamate in Zaydiyya branch of Shiism derives from the two groups views with regard to the legitimacy of the rule (imamate) of the caliphs preceding ʿAli, and the significance of the special knowledge attributed to the ahl al-bayt.
The former (Batriyya) denied the legitimate rights of the Ahl al-Bayt. They also held that though ʿAli was the most excellent (al-afdal) of Muslims to succeed the Prophet, nevertheless the caliphates of his predecessors who were less excellent (al- mafdul) were valid, because ʿAli himself had pledged allegiance to them. They also allowed the use of individual reasoning (ijtihad or raʾy) in religious matters in order to establish legal precepts.
Since the Batriyya were closely affiliated to the Kufan traditionist school, and after the latter’s absorption into Sunnism in the 3rd/9th century, the Batri Zaydi tradition also disappeared which led to the prevalence of the views of the Jarudiyya, the other sect in the current beliefs regarding imamate in Zaydiyya branch of Shiism.
Jarudiyya rejected the legitimacy of the caliphate of the caliphs before ʿAli. They held that the Prophet himself had designated ʿAli by the rule of the nass as his legatee (wasi) and implicitly as his successor The Jarudiyya ascribed superior knowledge to the ahl al-bayt in religious matters. However, in contrast to the Imamiyya, they did not confine legal teaching authority to their imams only, but accepted in principle the teaching of any member of the ahl al-bayt qualified by religious learning.
Zaydis did not generally consider their imams as divinely protected and immune from error and sin (maʿsum) and this is one of the most noticeable differences between the concept of imamate in Zaydiyyah branch of Shiism and that of the twelvers. The former later attributed such immunity (ʿisma) to the first three imams.
Zaydis did not recognize a hereditary line of imams, nor did they attach any significance to the principle of the nass, central to the Imami doctrine. Initially, as noted, the Zaydis were prepared to accept any member of the ahl al bayt as an imam, but later the imams were restricted to the Fatimid ʿAlids, descendants of al-Hasan and al-Husayn
They also rejected the eschatological idea of the concealment (ghayba) of an Imam-Mahdi and his return (rajʿa) in the future, and, in fact, messianic tendencies remained rather weak in Zaydi Shiʿism.
Taken from: A History of Shiʿi Islam
by: Farhad Daftary