Although he did not run for office, his coalition has received far more votes than current Iraqi Prime Minister Haider al-Abadi's political block, initial election results show.
With results counted in more than half of Iraq's 19 provinces, electoral officials said the second-largest number of votes went to a political alliance led by Hadi al-Amiri, a former militia leader with strong ties to Iran. While Sadr is unlikely to totally disavow any of Iraq's allies, both the USA and Iran will be sad to see more easily controlled leaders replaced with more independent ones, and that could have a long-term drag on relations.
An official at the US State Department remained coy ahead of the definitive tally, telling AFP "we are awaiting the announcement of the official results and look forward to the formation of the new government".
Iraqi Shiite cleric Muqtada al-Sadr has emerged as the biggest victor in parliamentary elections, limiting the chances for Prime Minister Haider al-Abadi to form another government and setting the country on an uncharted course.
Turnout was 44.52 percent with 92 percent of votes counted, the Independent High Electoral Commission said - that was significantly lower than in previous elections. Al-Abadi has repeatedly warned that unless Iraq forms a post-war government capable of attracting major investment, jihadists could mount a comeback in parts of Iraq.
In what could be a shock to Iraq's political system 15 years after the US invasion, the firebrand Shiite cleric Moqtada al-Sadr appears to have gained a large number of votes in Saturday's parliamentary elections, potentially placing him in a kingmaker role as the major winners of the vote try to piece together a governing coalition over the next few weeks. Former prime minister Nuri al-Maliki, a close ally of Iran like Amiri, came in fourth with around 25 seats.
The ranking of these blocs can still change with results yet to be announced from eight provinces, including Nineveh, which has the second-largest number of seats after Baghdad.
"He chose to surround himself with Dawa people", said political analyst Hisham al-Hashimi.
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In Sadr City, a poor, sprawling area of eastern Baghdad, Ramadan Mohsen, 50, told the Associated Press he had cast a blank vote because "the candidates have not done anything for the people".
Al-Abadi directed Iraqi forces to retake the city late past year after the Kurdish regional administration organized a referendum on independence that controversially included Kirkuk; federal forces moved in with little bloodshed as Kurdish forces withdrew.
The nationwide popular vote does not directly correspond to the amount of seats each list gains in parliament.
"We support a fair and transparent process", he said.
Sadr derives much of his authority from his family. If it can summon the focus and diplomatic resources. there's an opportunity to reinforce the incipient shift toward nonsectarian politics built around support for Iraqi sovereignty - including from Iran.
Tensions in the region have mounted - and in particular between the United States and Iran - partly because of President Trump's decision last week to withdraw the United States from the nuclear deal with Tehran. His father, highly respected Grand Ayatollah Mohammed Sadeq al-Sadr, was killed in 1999 for defying Saddam Hussein.
Abadi, preferred by the USA and credited for overseeing the crucial battle against Daesh as well as curtailing Kurdish independence ambitions, spearheads the Victory (Nasr) list.