BAGHDAD, Iraq — American troops are conducting security sweeps in the Shiite stronghold of Sadr City for the first time in three years, part of a revamped plan to pacify the capital. Yet the Mahdi Army militia of Shiite cleric Muqtada al-Sadr has not risen up to fight them, despite American raids on their homes and growing Sunni attacks on Shiites.

“Until now, our leader has ordered us to keep quiet,” a local official in Mr. Sadr’s organization, Ayad al-Khaby, explained. “This is in order for the security plan to succeed.”

After four years of hostility, Mr. Sadr and the Americans are cooperating uneasily as America and Iraq attempt to tame Baghdad’s sectarian violence. American officials, who in recent months described his Mahdi Army and other Shiite militias as the biggest threat to Iraq’s stability, now praise the Shiite cleric.

The collaboration represents a remarkable shift for two adversaries who control the largest armies in Iraq and who fought some of the fiercest battles since the 2003 U.S.-led invasion.

For Mr. Sadr, it is the latest stage in an evolution from populist cleric to guerrilla fighter to political kingmaker and now to powerbroker. In the early months of the occupation, U.S. officials dismissed Mr. Sadr as irrelevant to Iraq’s future. Today, they view him as a political catalyst who can help keep Iraq together — or implode it.

“We’re very encouraged by what we’re seeing on the ground right now in Sadr City,” the American military’s chief spokesman in Baghdad, Major General William Caldwell, said. “There is a tremendous amount of cooperation and dialogue ongoing. It’s proven to be very beneficial to both sides.”

It is a tenuous cooperation that could collapse at any moment. American troops walk a thin line between peace and war in Sadr City, a sprawling jumble of narrow streets, tan buildings and crowded markets. Each day tests the tolerance of Mr. Sadr and his fighters, who are widely believed to operate death squads. American commanders concede that their troops may face isolated attacks.

“They are an occupation force. We refuse their presence totally,” said Mohammad Abu Haider, a Mahdi Army commander who has battled Americans. “Their ultimate goal is to destroy the Sadr trend.”

Yesterday, gunmen ambushed the convoy of Sadr City’s mayor, Rahim al-Darraji, seriously wounding him and killing two of his bodyguards. Darraji, a Sadr appointee, has been negotiating with American and Iraqi government officials over the role of U.S. troops in the security clampdown.

A few hours earlier, at a luncheon with Western journalists, Major General Joseph Fil, the American commander in charge of Baghdad, spoke about Darraji.

“We’re in Sadr City, working closely with the mayor and it’s been completely permissive. It’s a collaboration,” he said.

Publicly, Mr. Sadr has criticized the American presence inside his stronghold. He is a fierce nationalist who has long demanded a withdrawal of U.S. troops from Iraq, and his authority derives in large part from his opposition to the occupation. But privately, he has ordered his militiamen to lay low no matter how much they are provoked by American forces, said fighters and Mr. Sadr representatives in interviews.

The absence of full-blown resistance against U.S. troops and the recent decline in the number of bodies found with signs of torture, usually attributed to the Mahdi Army, suggest that Mr. Sadr still controls the bulk of his forces, even as U.S. intelligence officials assert his grip over the Mahdi Army is slipping.

Such murders have dropped by a third during the first month of the security plan, General Fil said.

“If the Sayyid says it is in the best interest to rise up against the Americans, we will rise. If the Sayyid say there’s no interest, we won’t rise,” Mr. Haider said, using the honorific for a descendant of the Prophet Muhammad. “All the people in this city follow Sayyid Muqtada Sadr.”

After the invasion, Mr. Sadr channeled the growing disenchantment against the occupation, attracting poor, young and dispossessed Shiites into his militia. In 2004, Mr. Sadr’s forces staged two major uprisings against American forces. Since then, the Mahdi Army has repeatedly attacked U.S. troops and Iraqi security forces.

Even as his soldiers fought, Mr. Sadr sought political influence, focusing on the January 2005 elections. Today, his loyalists control 30 seats in the Iraqi parliament and four ministries. His support enabled Iraqi Prime Minister Maliki, a conservative Shiite, to enter office.

Despite intense American pressure, Mr. Maliki refused to take stern actions against his benefactor, preferring a softer approach. He publicly rebuked American raids into Sadr City. In October, he ordered American forces to lift a blockade of the area.

As a key component of the new security plan, Mr. Maliki proposed to American officials that he would persuade Mr. Sadr to order his militiamen to stand down, according to two Maliki aides. In return, he asked American forces to focus their efforts on combating Sunni insurgents, which the Shiite-led government views as the roots of the sectarian violence. U.S. commanders have publicly said they do not favor one sect over the other.

The top American military leader in Iraq, General David Petraeus, said in an interview that Mr. Maliki reached out to Mr. Sadr and his advisers. American and Iraqi commanders, he said, promised Mr. Sadr’s representatives they would enter Sadr City in “a respectable manner.”

Mr. Sadr’s cooperation “certainly has been a factor in the way we’ve been able to go into Sadr City, this early, this quickly,” General Fil said. “We were planning to go in later.” He described the ongoing sweeps in Sadr City as “gentle, cordon-and-knock type operations.”

America is funding 16 reconstruction projects inside Sadr City, although some were launched before the new security plan, the American Embassy charge d’ affaires, Ambassador Daniel Speckhard, said.

“The Americans are wise this time,” a political analyst, Wamidh Nadhmi, said. “When they first arrived they didn’t consult or have a dialogue with Sadr when he was the most important person in Iraq. Now, they are moving gradually, step by step, not very provocative as they had done in 2004.”

Mr. Sadr has other motives for allowing American soldiers into Sadr City, U.S. military officials said. In recent months he has become increasingly concerned about his political and religious image, because the Mahdi Army has been linked to torture and other crimes. He has purged militiamen from his fold and threatened to excommunicate others. He has also ordered his photos be taken down from government ministry offices he controls to discourage officials from justifying their actions by invoking his name.

American intelligence officials say he is competing for authority with extremist figures inside Mahdi Army who oppose his decision to join mainstream politics. By allowing American forces to enter his stronghold and arrest his militiamen, Mr. Sadr appears to be ridding his army of rogue fighters. In the past six months, nearly 700 of the “real extremist elements” of the Mahdi Army have been taken into custody and detained, including those who have committed death squad killings, General Caldwell said.

General Petraeus offered another reason for why the Mahdi Army has stood down so far. Many of Mr. Sadr’s advisers and Mahdi Army leaders have fled Sadr City to evade arrest, leaving rank and file fighters rudderless.

Despite the positive signs, U.S. generals remain skeptical.

“We have to be very cautious and expect there may be some setbacks,” General Fil said, acknowledging that the Mahdi Army may be waiting for an opportunity to attack. U.S. military officials are keeping a close eye on Mr. Sadr’s whereabouts. They say he is in neighboring Iran, but his followers insist he is in Iraq.

Any success in Sadr City, and other parts of Baghdad, hinges on not provoking Mr. Sadr and his loyalists. That is why, General Petraeus said, it was important for Iraqi soldiers and police the other day to stop a suicide bomber from entering Sadr City.

Sunni insurgents, he said, “will continue to try to create sensational attacks that could cause people to say `See, we need to have the Sadr militia to protect us again.”‘

But the collaboration remains controversial. The American military is building a garrison inside a police station to house U.S. and Iraqi troops on the fringes of Sadr City. It will become part of a constellation of neighborhood security outposts — the lynchpin of the new security plan to regain control of the streets.

“It’s a wrong idea. Sadr City has faced a lot of violations by the American forces,” a Sadr parliamentarian, Falah Shanshal, said. “There is no reason for the existence of such a base or the existence of Americans in the city.”

Said Mr. Haider, the Mahdi Army commander: “We always keep our eyes on the occupiers. We are never away from our duty.”