"The reforms are main goal, and the no-confidence vote is the beginning of the reforms," al-Sadr said in a rare and wide-ranging hour-long news conference. "And like ablution before prayers, reforms cannot happen without pressuring the government."

He added, "If the head is reformed, everything beyond it is reformed."

His declaration delivers a sharp blow to al-Maliki's efforts to hold on to power. Al-Sadr's followers are the first major Shiite party to take sides against al-Maliki.

Al-Maliki media adviser Ali al-Moussawi declined to comment on al-Sadr's statements. The prime minister's aides have previously predicted any vote to replace al-Maliki would fall short — as has Iraqi President Jalal Talabani, a Kurd.

Two other top Iraqi politicians — Kurdish regional president Massoud Barzani and the parliament speaker, Osama al-Nujaifi, a Sunni, are pushing for al-Maliki to resign.