BAGHDAD, Iraq, April 25 — A religious edict issued in Iran and distributed to Shiite mullahs in Iraq calls on them "to seize the first possible opportunity to fill the power vacuum in the administration of Iraqi cities." The edict, or fatwa, issued on April 8 by Kadhem al-Husseini al-Haeri, an Iraqi-born cleric based in the Iranian holy city of Qum, suggests that Shiite clerics in Iraq are receiving significant direction from Iran as they try to assert the power of Iraq's long-oppressed religious majority. It is not yet clear how much popular support Mr. Haeri and other clerics emerging...