2. Muqtada al-Sadr
Muqtada al-Sadr is the son and grandson of renowned Shiite clerics, and the family's popularity was clear the moment Baghdad fell in 2003, with the teeming Baghdad Shiite slum of Thawra (Revolution) immediately renamed "Sadr City" by residents. His base is very much the urban Shiite poor, and in his rivalries with other Shiite politicians, he emphasizes his ties to the working class and the fact that he never fled the country for comfortable exile under Saddam.
Middle East historian Juan Cole once described the movement around him as a kind of "Shiite Maoism," reflecting both his focus on the dispossessed and militancy. His Mahdi Army militia was ruthlessly efficient in cleansing parts of Baghdad and other cities of Sunni Arabs at the height of Iraq's civil war. Though the Mahdi Army is now formally disbanded, there's little doubt he can call on skilled gunmen if there's need for it.
Mr. Sadr kicked his 40 seats in parliament to Maliki's side in the battle to form the government in 2010, extracting eight cabinet seats in exchange that he's since used to generate money and support for his cause (Iraq's political parties use government ministries as fundraising and patronage vehicles).
This week, a top aide to Sadr called for new elections, charging Maliki is mishandling the country. Whether that's a serious demand, or a ploy to extract more cabinet seats from Maliki (who is mulling unilaterally dumping his Sunni ministers), remains to be seen. But Sadr's ultimate ambitions remain undiminished, and any alliances he makes will be uneasy ones.