A political outlier before Saturday's ballot, Sadr is best known for leading the "fearsome" Mehdi Army in two insurgencies against United States troops in Iraq, following the toppling of Saddam Hussein in 2003.
After being sidelined by Iranian-backed rivals for years, the apparent parliamentary victory marks a political comeback for Sadr, who didn't even officially run for prime minister in this year's elections.
Under article 76 of Iraq's constitution, the right to form a government falls to the political bloc with the most seats.
He can not become prime minister as he did not run in the election, though his apparent victory puts him in a position to pick someone for the job. The government should be formed within 90 days of the official results. The elections were held Saturday, with low turnout. The group overran a third of Iraq in 2014.
With Sadr's Alliance of Revolutionaries for Reform coalition [Saairun] in the lead as 16 of Iraq's 18 provinces are counted, it now appears the Shia cleric will become Iraq's main powerbroker.
Reports state that the alliance of the Sadr Movement and the Iraqi Communist Party won the approval of just over 1.3 million people and the gain of 54 seats in a parliament of 329. By the end of the announcement, al-Sadr's list had the highest popular vote, followed by al-Amiri's. Al-Sadr said in Tweet he was open to forming a coalition with al-Abadi to form a new government for Iraq.
But after 14 years, more than $1 trillion, millions of Iraqi dead and thousands of USA troops killed and maimed, the US now has less influence over Iraq than it had while former Central Intelligence Agency operative Saddam Hussein was in power. The other winning blocs would have to agree on the nomination.