WASHINGTON, Sept 12 (Reuters) - Democrat Barack Obama on Wednesday called for completing a U.S. withdrawal from Iraq by the end of 2008 and said if elected president he would start an effort to help Iraqis bridge sectarian differences.

"I am here to say that we have to begin to end this war now," the Illinois senator said in excerpts from a speech he was to deliver later in Iowa. The excerpts were released by his presidential campaign.

He said he would immediately begin to pull out troops engaged in combat operations at a pace of one or two brigades every month, to be completed by the end of 2008.

Obama also cautioned against dumping Iraqi Prime Minister Nuri al-Maliki, who is held in deep skepticism by many U.S. lawmakers.

"The problems in Iraq are bigger than one man," Obama said.

Obama was speaking after the top U.S. commander in Iraq, Gen. David Petraeus, told the U.S. Congress this week that he recommended gradually withdrawing up to 30,000 U.S. troops from Iraq by next summer, a proposal President George W. Bush is expected to endorse.

Obama said he would call for a new constitutional convention in Iraq, convened with the United Nations, which would not adjourn until Iraq's leaders reach a new agreement on political reconciliation.

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