Washington (CNN) Anti-American cleric Muqtada al-Sadr's coalition election victory in Iraq raises the prospect that the man many US military officials hold responsible for the deaths of US troops could become a kingmaker in Iraqi politics.

Al-Sadr has long positioned himself as an independent nationalist, a foe of both Iran and the US, both of which have sought to influence Iraq's trajectory since the 2003 invasion that toppled Saddam Hussein.

Al-Sadr campaigned on an anti-corruption platform, allied himself with the Communist Party and, like so many politicians across the world, rode a wave of populist sentiment to victory. He knocked out Washington's preferred candidate, Prime Minister Haider al-Abadi, who finished in third place.

While al-Sadr carries the weight of years of bad blood with the US, after leading two insurgencies against American forces in Iraq, some analysts say the math of Iraqi coalition building means he won't necessarily determine the shape of Iraq's immediate future.

If he does become Iraq's power broker, others say al-Sadr has developed a more pragmatic approach over time, noting his openness to the US military presence in Iraq to support the fight against ISIS.

Bad news for who?

"Is this bad news for Iran, or bad news for America?" asked Randa Slim, director of conflict resolution at the Middle East Institute. "It's bad news for both. I think there is a lot of concern, but if anybody has been following his political and ideological trajectory, this is a guy who has evolved into a pragmatic actor."

Slim noted that al-Sadr publicly said he was fine with a US presence in Iraq to fight ISIS and to equip and train Iraqi troops, as long as it didn't impinge on Iraqi sovereignty. About 5,000 US troops are stationed in the country.

Fifteen years after the US-led invasion of Iraq, officials in Washington reacted to the news about al-Sadr with neutral caution.

"The United States congratulates Iraq and its people for voting and participating in the democratic process," a National Security Council spokesman said Tuesday. "While the final vote tallies and seat allocations have not yet been certified, preliminary results demonstrate that Iraqis are eager to build a safe and prosperous future for themselves, in the context of a nationalist, centrist government that is sovereign and stable."

"We look forward to working with the new Iraqi government that emerges from the Iraqi constitutional process," the spokesman said, "to help deliver stability, security and prosperity for all Iraqis."

The cleric's victory could pose complications for President Donald Trump. Senior US troops and commanders feel al-Sadr is responsible for the deaths of many US troops and will be watching closely to see how much of a box Trump will be in, if the US does have to deal with him.

The US military focus on al-Sadr had been intense. In 2004, Lt. Gen. Ricardo Sanchez, the commander of coalition ground forces in the US-led occupation of Iraq, said that "the mission of the US forces is to kill or capture Muqtada al-Sadr. That's our mission."

At the Pentagon on Tuesday, Defense Secretary James Mattis was asked if he was angered by the election results, given al-Sadr's role in killing so many American and coalition troops. Mattis served in Iraq and headed US Central Command, which oversees the Middle East.

"The Iraqi people had an election," Mattis said. "That's a democratic process at a time when people, many people, doubted that Iraq could take charge of themselves. So we will wait and see the results, the final results of the election, but we stand with the Iraqi people's decision."

After the Iraq Electoral Commission finished more than 90% of the counting in 18 provinces and initial results showed al-Sadr's coalition in first place, Abadi called "on citizens and the political coalitions to respect the election results" in a televised speech broadcast on Iraqiya state TV.

Some US concern is driven by the dangerous undertow of Iraq's sectarian politics: For the first time since the fall of Saddam, Shiite Muslim parties are theoretically able to form a government without Sunni Muslim or Kurdish participation, said Michael Pregent, a senior fellow at the Hudson Institute.

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Pregent, a former intelligence adviser to Gen. David Petraeus and Gen. Ray Odierno in Iraq, says Sunni Arabs and the Kurds "have been the bulwark against Iran in the past, and against the existential threats of ISIS and al Qaeda."

If Shiite parties form a government without other groups, Pregent said, it would likely lead to continued instability in Iraq.

Sectarianism has been a major problem in Iraq. Under Saddam, majority Shiites were brutally repressed by minority Sunnis, who dominated politics and state institutions. With the 2003 invasion of Iraq and the fall of Saddam, sectarian violence exploded as Shiite leaders exacted revenge for decades of oppression.

Pregent alluded to the ties between some of Iraq's Shiite political parties and Iran.

"If you simply allow the Shiite political parties, the 60% population of Iraq, to form a government, we will continue to have an unstable northern Iraq where threats are incubated," Pregent said, "and Baghdad and Iran have no interest in stopping it because it justifies militias, it justifies Iran being in Iraq, it justifies Syria."

Slim points out that al-Sadr is no great fan of Iran and has already said he's not willing to form a government with pro-Iranian parties that would be problematic for Washington. "He's looking for a governing alliance with people who aren't seen maybe as pro-West allies but are seen as possible for the West and the US," Slim said, citing Abadi and Kurdish groups as possible partners al-Sadr might seek.

And like Pregent, she points out that in the fluid give-and- take of Iraqi politics, it's very possible that al-Sadr might not end up being Iraq's new power broker.

Indeed, pro-Iranian parties have begun working to try to outflank al-Sadr. Agence France-Presse has reported that the powerful Iranian general Qassem Soleimani met with Abadi and pro-Iranian politicians this week in a bid to create a bloc large enough to displace al-Sadr.