BAGHDAD, Iraq, March 27 - For several weeks, Iraq's most powerful politicians and foreign diplomats have been streaming like anxious pilgrims to western Baghdad, to the vast blue and gold dome of the Mother of All Battles mosque, which was commissioned by Saddam Hussein. They are there to visit Sheik Harith al-Dari, a 64-year-old cleric and tribal leader who has become a leading spokesman for Iraq's disaffected Sunni Arabs. Mr. Dari, a taciturn man with an air of cold authority, greets his guests in a dim office off the mosque's main hall, which is surrounded by a moat and tall...