BAGHDAD — Moktada al-Sadr, a firebrand militia leader whose forces once battled American troops in Iraq and were implicated in widespread atrocities against civilians, has emerged as the surprise front-runner in the Iraqi national elections, according to Iraqi election officials.
After American forces withdrew from Iraq in 2011, Mr. Sadr remained vocally anti-American, though he has also been strongly critical of Iran, the other foreign power with widespread influence here.
The victory of Mr. Sadr’s political coalition could complicate the American strategy in Iraq. The American military has been training, sharing intelligence and planning missions with former militias in the country, gambling that their military partnership can keep the Islamic State from making a comeback here.
Mr. Sadr has been highly critical of American airstrikes in the country against the Islamic State, though he has said little recently about his willingness to allow American troops to remain on Iraqi soil.
American officials are now uncertain — though not yet worried — about what the position of Iraq’s future government may be on the issue.
Some of Mr. Sadr’s political allies, even those who fought against American soldiers in the past, want the United States to stay and help shore up the country. His closest rivals in the election also support the Americans staying. And even Mr. Sadr’s representatives have said that he would abide by agreements between the United States and Iraq on training Iraqi security forces.
Mr. Sadr once led the Iraqi Shiite militia known as the Mahdi Army, which fought with American forces after the 2003 invasion that toppled Saddam Hussein and the Sunni-led minority in power then. The Mahdi Army was blamed for death squads that committed reprisals on Sunnis during the country’s worst sectarian convulsions in 2006 and 2007.
Early in the American-led invasion, President George W. Bush called Mr. Sadr an enemy and even briefly considered the idea of having the American military capture or kill him. “We can’t allow one man to change the course of the country,” Mr. Bush said in a video teleconference.
Mr. Sadr, a scion of a Shiite clerical family, has spent years refashioning his image. He now offers himself as a populist outsider intent on fighting corruption, and he has capitalized on the grievances shared by many Iraqis over the systematic graft in their country.
He no longer rails so defiantly at America as he once did, when his disciples killed United States soldiers and committed atrocities against Sunnis in sectarian bloodletting. He has cast himself as an ardent nationalist, and distanced himself from Iran, the Shiite power next door that established a growing influence in Iraq after the American military withdrawal in 2011.
While Mr. Sadr was once known for prolonged trips to Iran, he raised eyebrows less than a year ago with a rare visit to Saudi Arabia, Iran’s regional rival, where he met with Crown Prince Mohammed bin Salman and others.
Mr. Sadr’s Shiite militia fighters played a prominent role in helping in the fight to evict the Islamic State extremists who seized major Iraqi cities four years ago, and many are now part of Iraq’s regular armed forces.
In a 2017 interview with a Turkish television station, Mr. Sadr advocated reaching out to Iraqi Sunnis in Mosul who had abided by the militant group’s calamitous occupation of that city.
“There are still moderates among the people but they are scared,” he said. “We have to give them a chance to show up and give their ideas.”
Now, he has upset the nation’s political order.
Although Mr. Sadr was not a candidate and cannot become prime minister himself, he led an unlikely alliance of candidates that included Iraq’s moribund Communists, Sunni businessmen and pious community activists who have gained a following for their strong stance on corruption.
Mr. Sadr’s coalition, Iraqi officials say, placed first in six provinces, including the densely populated capital, Baghdad. His alliance could have the first opportunity to build a governing coalition, and on Monday night in Baghdad, crowds of youths in the city’s impoverished Sadr City area waved pictures of Mr. Sadr and set off fireworks.
But there is no guarantee that Mr. Sadr will have the final say on who Iraq’s next leader is, or even a major one. After final results are announced, negotiations between Iraq’s competing parties are expected to continue for the next 15 days, meaning the political landscape could well change.
Mr. Sadr’s surprise election victory suggested that Iraq itself may be changing, too.
Since the first elections after the fall of Hussein in 2003, Iraqi voters have rarely strayed from their political bases, dutifully supporting candidates representing narrow religious or political beliefs. But the vote over the weekend offered evidence that populism may be replacing sectarianism as the defining force in Iraqi politics.
In this election, many voters abandoned their traditional divisions and supported two new political movements groups that promised to tackle a pervasive everyday problem: corruption. The groups also ran on a pledge of “Iraq First” — a rebuke to the outside powers many blame for recent instability, namely Iran and the United States.
“I expected that voters would punish the big political blocs for their failures,” said Hussein Allawi, a political science professor at Al Nahrain University in Baghdad. “Their election campaigns offered nothing to voters and were unconvincing.”
The winners not only scramble the pyramid of power in Iraq but also raise the possibility of a government with radically new priorities.
Margaret Coker reported from Baghdad, and Rick Gladstone from New York.