Sadr is the son of a well respected cleric murdered by Saddam's regime. When the Coalition invasion of Iraq reached Baghdad Sadr quickly organized his father's followers and had them distribute food and implement security before the Coalition did. This created a power base among the urban poor Shia of Baghdad and nearby areas. In 2004 Sadr led two open rebellions against the Coalition forces, continued on-again off-again military attacks, and yet managed to become active in the political process.
Fortune began to turn against Sadr in 2007; however. Sadr left before the "Surge" could get to him and his militia split between various groups. The Coalition and Iraqi forces step up action against the Mahdi Army and Iranian-backed rogue elements known as Special Groups.
In response to their loses the Mahdi Army lashed out at enemies assassinating opponents even, possibly, a Shia Badr-backed governor. Killing Iraqis though is unpopular and counter-productive as al Qaeda has found out. The final straw was this weekend when Mahdi Army elements went on a rampage killing Badr militia and other Shia pilgrims in Karbala. These thuggish actions are costing Sadr the support he once enjoyed so he is "suspending" Mahdi Army activity. But with the lack of control of all of the Mahdi Army it waits to be seen if Sadr is still the major player he once was.