Sadr himself cannot become prime minister as he did not run in the election, though his bloc's victory puts him in a position to have a strong say in negotiations. His Sairoon electoral list captured 54 parliamentary seats.
The Victory Alliance, headed by incumbent Prime Minister Haider al-Abadi, trailed in third place with 42 seats, several seats behind the Al-Fatih bloc, which won 47 seats.
The victory was a surprising change of fortunes for the Shi'ite cleric. Sadr, who made his name leading two violent uprisings against US occupation troops, had been sidelined for years by Iranian-backed rivals.
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