BAGHDAD (Reuters) - The leader of one of Iraqs most powerful Shia political groups and most important religious dynasties died on Wednesday, adding to political uncertainty in a violent run-up to an election next January. Abdul Aziz al-Hakim, who headed the Supreme Islamic Iraqi Council (ISCI), a major partner in the Shia-led government, died while undergoing treatment for cancer in Iran, ISCI said. It is a painful event and a great tragedy, the ISCI-owned television station quoted Ammar al-Hakim, his son and likely successor as party leader, as saying. ISCI officials said two funerals would be held, in Iran and in Iraq. Born in 1950, Hakim lead ISCI since 2003 after his brother, Grand Ayatollah Muhammad Baqer al-Hakim, was killed by a car bomb. ISCI is part of Iraqs ruling Shia alliance, which includes Prime Minister Nuri al-Malikis Dawa party, but it said this week it would lead a new group to compete in Januarys polls without Maliki. Hakims death at this sensitive stage that we are going through is considered a big loss, Maliki said in a statement. Anti-American cleric Moqtada al-Sadr, a fellow member of the new, mainly Shia alliance said: This is a promise from me to all his be brothers and partners in this life and the next as they ask for the liberation of Iraq. State television displayed a black banner of mourning and passages from the Quran in Hakims honour. Political analyst Hazim al-Nuaimi said the loss of ISCIs chief peacemaker could lead to infighting. ISCI must take care to line up behind the new leader, whoever he may be, in the five months before what are sure to be fiercely contested elections. Anyone who sees ISCI as vulnerable will try to take its place, said Mohammed Abdul Jabar, a former Shia politician who now edits a weekly magazine. Jalal al-Din al-Sagheer, who heads ISCIs parliament bloc, said Ammar al-Hakim would be interim party chief and that a permanent leader would be chosen soon by senior clerics.