A looming split between the two Shiite parties that dominate Iraq's government threatens efforts to win parliamentary approval for a security pact with the U.S. and could set the stage for a major struggle for power in the oil-rich Shiite southern heartland.

Prime Minister Nouri al-Maliki's Dawa Party and the Supreme Islamic Iraqi Council led by Abdul-Aziz al-Hakim have been allies since the 2003 fall of Saddam Hussein's Sunni-led regime.

Now they are rapidly turning into bitter rivals, raising the specter of a weakened Shiite front ahead of two key elections next year.

The security agreement, reached after months of tortuous negotiations, would allow U.S. troops to remain here after their U.N. mandate expires Dec. 31. It is critical to ensuring Iraq's security until government forces are capable of taking charge of the fight against insurgents.

A draft has been completed and the government is preparing to submit it to parliament for final approval — which U.S. officials believe is by no means certain.

Although passage would require only a majority of the 275-member parliament, al-Maliki will submit the draft only if he is convinced it will receive two-thirds support — which would allow him to fend off critics both here and in neighboring countries such as Iran and Syria, according to al-Maliki's aides, who spoke on condition of anonymity because they were not authorized to discuss strategy.

To reach two-thirds, the draft would need the 30 votes from the Supreme Council.

Secretary of State Condoleezza Rice discussed the pact over the telephone late Wednesday with Vice President Adil Abdul-Mahdi, a top member of the Supreme Council. (In Washington, Defense Secretary Robert Gates was calling U.S. congressional leaders in support of the agreement.)

But the Supreme Council has said little in public about the negotiations, a stand that a senior aide to party leader al-Hakim said was designed to distance the party from the agreement if it meets significant opposition.

"The Supreme Council did not want to be associated with the agreement," said the aide, who spoke on condition of anonymity because of the sensitivity of the subject. "If it founders, then al-Maliki alone must deal with the consequences."

The first hint of opposition by the Supreme Council to the agreement emerged Thursday.

Senior lawmaker Jalal Eddin al-Sagheer said the party will seek "clarification" from al-Maliki when he meets with parliament leaders over the weekend.

Another Supreme Council legislator, Diaa Eddin al-Fayadh, said the party planned to vote on the agreement at a later unspecified date.

Even if the Supreme Council stands behind al-Maliki on the agreement, the growing split between the two Shiite parties could threaten the political stability that the U.S. military believes is vital to maintaining fragile security gains made over the past year.

Politicians from both groups say they expect the dispute to endure through the provincial elections expected by Jan. 31 and a general election before the end of 2009.

The two parties have always been uneasy partners, forced into an alliance because they needed to cement the political gains won by the long-oppressed Shiites after Saddam's fall.

The Supreme Council was created in Iran in the early 1980s and has since maintained close ties to Tehran. What it lacks in popularity it makes up for with good organization and extensive funds.

Dawa, the older of the two, has traditionally attracted more secular and educated Shiites and, unlike its rival, keeps some distance from Iraq's powerful clerics and Iran's ruling clergy.

Relations between the parties began to sour after al-Maliki grew stronger and more assertive following a series of political and military successes that coincided with a dramatic reduction in violence following last year's U.S. troop buildup.

Ties further strained after al-Maliki sought to compete with the Supreme Council for influence in southern Iraq, luring tribesmen with cash and jobs to form "Support Councils" — government-backed groups designed to extend support to security forces in their provinces.

The Supreme Council hit back with rare criticism of al-Maliki. Al-Hakim's son and apparent heir, Amar al-Hakim, suggested in a recent speech that the prime minister, whom he did not mention by name, was using oil revenues to woo voters with cash donations.

The party also ordered provincial governments it dominates in southern Iraq not to cooperate and in some cases disband the government-backed tribal groups al-Maliki set up.

"We told them that these groups are meant to win votes for Dawa," said al-Fayadh, the Supreme Council lawmaker. "We want them dissolved."

The Supreme Council's concern about Dawa's moves in southern Iraq reflect the party's resolve to maintain its hold on the region in the January provincial vote so it can press ahead with its plans to create a self-rule region there similar to the 17-year-old Kurdish entity in the north.

Most Shiite parties, including Dawa, oppose the move, as do Sunni Arabs who fear it would lead to the breakup of Iraq.

Once dismissed as a stopgap figure, al-Maliki has developed into a national leader, threatening the Supreme Council's ambition to win the top job after next year's elections.

With its 30 seats in parliament — five more than Dawa has — it feels it has a legitimate claim to the post.

To prevent that, al-Maliki may soon announce an alliance between his party and the estimated 30 independent Shiite lawmakers to contest next year's balloting.

Signs also are emerging that al-Maliki has been slowly mending fences with the 30-seat bloc in parliament that is loyal to radical cleric Muqtada al-Sadr as well as the 44-seat Sunni Arab alliance.

Last year, both quit al-Maliki's government over various differences.