Baghdad, Iraq ? Iraqi negotiators reached a breakthrough deal on the constitution Tuesday, and at least one Sunni Arab party said it would now urge its followers to approve the charter in this weekend’s referendum. Suicide bombings and other attacks killed more than 50 people in the insurgent campaign aimed at intimidating voters.

Under the deal, the two sides agreed on a mechanism to consider amending the constitution after it is approved in Saturday’s referendum. The next parliament, to be formed in December, will set up a commission to consider amendments, which would later have to be approved by parliament and submitted to a referendum.

The agreement boosts the chances that the draft constitution will be passed in Saturday’s nationwide vote. Shiite and Kurdish leaders support the draft and the United States has been eager to see it approved to avert months more of political turmoil, delaying plans to start a withdrawal of U.S. forces.

In return, the agreement guarantees Sunni Arabs the ability to try later to introduce major changes they want, aimed at reducing the autonomous powers that Shiites and Kurds would have under the federal system created by the charter, negotiators said.

“The important principle here is that this provides an assurance (to Sunnis) that this constitution is not the end of history but is subject to amendment,” said Ridha Jawad Taqi of the Supreme Council of the Islamic Revolution in Iraq, a top Shiite party in the government.

U.S. officials have pushed the three days of negotiations between Shiite and Kurdish leaders in the government and Sunni Arab officials, that concluded with marathon talks at the house of President Jalal Talabani late Tuesday.

A Kurdish worker from Irbil loads his truck Tuesday with posters, mostly in Kurdish language, in favor of the new Iraqi constitution, to be distributed in different districts and governorates inhabited by Kurds in Irbil.

A top Sunni negotiator, Ayad al-Samarraie of the Iraqi Islamic Party, said the measure would allow it to “stop the campaign rejecting the constitution and we will call on Sunni Arabs to vote yes.” It was unclear if parliament would formally vote on the new deal with some lawmakers saying that measure may be read to the National Assembly today.

But other major Sunni parties were not present at the negotiations, and at least one senior Sunni leader, Saleh al-Mutlaq, said he was not yet convinced by the measure.

“The Islamic party was participating alone in these negotiations and making its own decisions,” al-Mutlaq said. “This is strange because the Iraqi Islamic party does not represent all the Sunni Arabs but only a small percentage of them.”

But the announcement was the first break in the ranks of Sunni Arab leaders and will likely deeply undermine the campaign to defeat the constitution at the polls. Sunni-led insurgents have demanded a boycott of the election and threatened those who would vote.

The agreement outlines four additions to the document considered Saturday that outline how future amendments will be made.

The central addition allows the next parliament, which will be formed in Dec. 15 elections, to form the commission, which will have four months to consider changes to the constitution. The changes would be approved by the entire parliament, then a referendum would be held two months later.

That is no guarantee that Sunnis will be able to make the changes they seek. They are likely to have a stronger representation in the next parliament, but would still face a strong Shiite and Kurdish majority that would likely oppose major changes.