"If reports that (Abdul-Aziz al-) Hakim (leader of the Supreme Council for the Islamic Revolution in Iraq) is truly ill with lung cancer are true, this could seriously shift the balance of power," Anthony H. Cordesman, who holds the Arleigh A. Burke chair in strategy at the Center for Strategic & International Studies, a Washington think tank, said in a statement.

"SCIRI does seem to be losing political influence and strength in the oil-rich southeast, while Sadr's Mahdi Army remains a major force. Coming back (to Iraq from Iran) allows him to reassert control and game U.S.-led security operations," Cordesman said.

"The Sadrists have also shown that they can cooperate just enough with the United States and the Iraqi Security Forces to take the credit for improvements for local security in the areas where the Sadr militia already plays the role, and get away with claiming the credit for any successes in aid while still blaming the United States and government for the overall lack of progress," the analyst said.

"At a different level, Sadr may also feel (Grand Ayatollah Ali al-)Sistani and the traditional clergy are losing influence and relevance as Iraq's sectarian violence rises and 'Quietism' has failed to change Iraq," Cordesman said. "It is important here to remember that Sadr does not need the support of the Shiites who do not act, only those who will be politically active and use violence. These are younger, more urban, and far less tied to tradition."

Sadr has returned to Iraq from Iran and has indicated he is prepared to take a more active role again in Iraqi politics.