BAGHDAD — A once-feared Shiite militia that was crippled two years ago by defections and a U.S.-Iraqi crackdown has quietly started to regroup, adding street muscle to the Shiite party that emerged strongest from Iraq’s parliamentary elections.

The revival of the Mahdi Army, loyal to radical cleric Muqtada al-Sadr, could be an ominous sign. An al-Sadr spokesman says the force is gearing up to ensure U.S. forces stick to a Dec. 31, 2011 deadline to withdraw from the country — threatening attacks on American troops if they stay past the date.

In the near-term, Sunnis fear the militia will turn its firepower against their community in vengeance after an uptick in militant violence against Shiites in recent months, a move that could revive the fierce sectarian bloodshed that nearly tore the nation apart in 2006 and 2007.

Al-Sadr disbanded the militia in 2008. But his spokesman, Salah al-Obeidi, told The Associated Press that it has now officially been revived.

The militia’s armed wing, called the “Promised Day Brigade,” will “prepare quietly to launch qualitative attacks against the occupiers (U.S. forces) if they stay beyond 2011,” he said. “It will have a big role to play to drive them out of Iraq.”

In a show of the movement’s new boldness, al-Sadr offered to help Iraqi security forces — who have almost no visible presence in their eastern Baghdad stronghold — protect Shiites after a wave of bombings April 23 targeted their places of worship. Prime Minister Nouri al-Maliki did not respond, and a top aide, Ali al-Adeeb, expressed doubt that the government would accept the offer.

So al-Sadr took matters into his own hands. Last Friday, his militiamen deployed at the sites of the weekly Muslim prayers organized by the Sadrists in Baghdad’s Sadr City — home to some 2.5 million Shiites — and across the Shiite south of Iraq, throwing a security ring around their mosques, searching worshippers and vehicles.

The Mahdi Army’s return comes during a dangerous political vacuum resulting from the inconclusive March 7 vote. No political bloc emerged with the parliamentary majority needed to form a new government, sparking wrangling between al-Maliki and his top rival, Ayad Allawi, that is likely to last for weeks, maybe months. Similar deadlocks have in the past coincided with a marked rise in violence.

The Sadrist movement made considerable gains in the election, winning 40 of the legislature’s 325 seats, the largest number by a single Shiite party. As a result, the Sadrists could hold the role of kingmakers in a new, Shiite-led government.

Set up in 2003, the Mahdi Army rapidly grew into the primary Shiite force in Iraq during the hardest years of the Sunni-led insurgency. It protected Shiite neighborhoods, and was believed to have been behind many of the sectarian slayings of Sunnis during the Shiite-Sunni violence of 2006 and 2007. It also fought U.S. forces in two major uprisings.

In 2008, U.S.-backed Iraqi forces drove its fighters off the streets of Baghdad and southern cities. By the time of its defeat, many of the militia’s neighborhood vigilantes dabbled in protection rackets, black marketeering and kidnaping for ransom. Others have enforced a strict interpretation of Islamic law on Shiite residents, shutting down music stores, hair dressers, bombing liquor shops and forcing women to cover up head-to-toe in public.

After disbanding the militia, al-Sadr sought refuge in neighboring Iran, moves that plunged the Mahdi Army into disarray. The more militant of its fighters broke away and formed Iranian-backed cells to attack U.S. forces.

Al-Obeidi, the spokesman for al-Sadr, said that besides the armed wing, the militia has two other divisions — the “Momahedoun,” or those who pave the way, and the “Monaseroun,” the loyalists, which respectively focus on religious indoctrination and the mobilization of supporters.

A Mahdi Army commander said the military wing now boasts several thousand fighters, though he refused to give a specific number. He said it has a strict code of conduct and secrecy, barring fighters from revealing their members on pain of expulsion.

“There is more discipline now than at any time since the Mahdi Army was formed and everyone will have to abide by the new rules,” said the commander, who spoke on condition of anonymity because he was not authorized to share the information with the media.

The assertions by al-Obeidi and the commander could not be independently verified. But besides the show of force Friday, there have been other signs of the militia’s re-emergence.

Mahdi Army militiamen in their trademark black shirts have taken to parading again on the streets of Shiite neighborhoods in Baghdad and elsewhere. Some of them perform the prayers wearing white shrouds, signaling their readiness for martyrdom.

In the mainly Shiite port city of Basra in southern Iraq, there have been a series of recent attacks on liquor stores and a number of unresolved murders of security officers thought to have been involved in the ill treatment in detention of Mahdi Army members. Security officials speaking on condition of anonymity for fear of retaliation say the attacks bear the hallmarks of the Mahdi Army.

So far, Shiites have not retaliated unilaterally for al-Qaida attacks on their community, which have persisted even though violence in general around the country has been down for more than two years.

But many fear that could change if the Mahdi Army emerges as the self-styled protectors of the Shiites.

“There is a legitimate security force in this country called the Iraqi security forces,” U.S. military spokesman Maj. Gen. Stephen Lanza said when asked about the Mahdi Army’s plans. There is no “legitimate role” for militias in Iraq now.

A senior U.S. military official speaking on condition of anonymity due to the sensitivity of the situation downplayed worries of a Mahdi Army resurgence, describing their capabilities as more “disruption” than anything else and that Americans have seen few signs they were regrouping.

But senior Sunni politician Salim Abdullah warned that if militias start taking the role of the country’s military and police, it “will lead to chaos.”

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Hendawi reported from Cairo. Associated Press writers Rebecca Santana and Sameer N. Yacoub contributed to this report from Baghdad.

CORRECTS Lanza’s rank to major general, transliteration of Allawi’s first name.