Abadi could be out of a job if Sadr, reportedly closer to Saudi Arabia than Iran, allies himself with Iranian proxies, such as Hadi al-Amiri.
Sadrs ticket won the most seats in Iraqs parliamentary election, according to results from all 18 provinces released Monday, placing him in the best position to select the next prime minister and set the course for how the nation emerges from a costly war against the Islamic State. He and former Prime Minister Nouri al-Maliki, both Dawa officials, ran on separate candidate lists, splitting the vote for the party.
People in Iraq voted Saturday in the first parliamentary elections since the country declared victory over ISIS terrorist group.
An electoral alliance of Hashd-linked candidates, headed by militia commander Hadi al-Amiri, is now in second place in the election returns.
The Iraqi air force has already carried out several air strikes against Islamic State in Syria since previous year, with the approval of the Syrian government of President Bashar al-Assad and the US -led coalition fighting Islamic State.
The results unexpectedly showed former Prime Minister Nuri al-Maliki, who was touted as a serious challenger to Abadi, lagging behind.
Abadi was viewed as a frontrunner before the election.
A Sadr victory or second-place finish would mark a surprise comeback by the cleric, who has a zealous following among the young, poor and dispossessed but has been sidelined by influential Iranian-backed figures. Iraq's many political factions mean a government may only be formed after drawn out negotiations.
Sadr derives much of his authority from his family.
His father's cousin, Mohammed Baqir al-Sadr, was killed by Saddam in 1980.
After the announcement that the Marching Towards Reform was ahead in Baghdad, supporters took to the streets in the capital to celebrate early Monday.
On Saturday, Iraqis voted in the country's first parliamentary election since 2014.
The nationwide popular vote does not directly correspond to the amount of seats each list gains in parliament.
The election came as the country deals with the disenfranchisement of the country's Sunni minority.
Sadr's ascendancy comes at the expense of incumbent Prime Minister Haider al-Abadi, the preferred candidate of the United States, who came in third overall.
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. He had no powerful political machine of his own when he took office. However, the Kurds will still have a key role to play in coalition forming and various Arab alliances will try to entice Kurdish support.
The Iraqi elections also serve as a battleground for the US and Iran, with Tehran continuing to wield significant influence over Baghdad. The dissident-turned-militia leader spent more than two decades fighting Saddam from exile in Iran. This opens the doors for various other blocs with a significant number of seats to align with Abadi and seek significant concessions in the process.