"Muqtada was not a proper cleric. To begin with, he was too young; his father's legacy gave him charisma and support but not the scholarly attainment and respect that are the real sinews of lasting esteem among the Shia clergy. It did not help that he had failed to finish his seminary education, and that as a youth he was better at playing video games than dealing with intricacies of Shia law and theology (in his seminary days he was nicknamed Mulla Atari, after the maker of electronic amusements). Like the scions of many other clerical families of Iraq, he found himself catapulted into a position of power and prominence because his father and older brothers had been killed. So weak were Muqtada's religious credentials that he had to rely initially on the authority of one of his father's allies, the Qom-based Ayatollah Kadhim Husayn al-Haeri, until al-Haeri, worried by Muqtada's erratic politics, found it prudent to distance himself.
What Muqtada laced in religious credentials he tried to make up for with a radical brand of politics exacerbated by his own unstable personality. He preferred fighting his battles in the political arena, and actors ranging from Hezbollah and Iran's Revolutionary Guards to Ahmed Chalabi manipulated him to serve their own ends. Even Sistani, who was a target of Muqtada's rash politics, found the young firebrand cleric a useful tool in dealing with the U.S. administration in Baghdad, since he and Muqtada formed a kind of de facto 'good cop, bad cop team that helped to keep the Americans off balance.
Muqtada's rebel image, mixing Islam and nationalism, and his willingness to challenge U.S. authority, gained him popularity. His movement, however, lacked coherence. It was powerful but chaotic, best characterized as street (or barrio) politics. He had a cultlike following among poor and uneducated Shia youth, and his support in southern Iraq grew after he threw his Mahdi Army into a fight against U.S. forces in 2004." -Vali Nasr, The Shia Revival