Iraq's top Shiite cleric urged unity on Friday after Nouri al-Maliki agreed to step down as prime minister, reviving hopes for a new government that can take on Sunni insurgents who have overrun large parts of the country. Al-Maliki announced he was giving up his post on national television late Thursday, standing alongside senior members of his Shiite Islamic Dawa Party, including his rival and premier-designate Haider al-Abadi.