Voters have a long list of grievances, ranging from poor public services to rampant corruption and high unemployment, but the month-long campaign has centred on Maliki's bid for a third term and a dramatic worsening in security in recent months.
More than 20 million voters will choose between upwards of 9,000 candidates running for 328 seats in parliament throughout the day, from the opening of voting at 0400 GMT until 1500 GMT.
The run-up to the election has seen Baghdad and other major cities swamped in posters and bunting. Parties have staged rallies and would-be lawmakers have angrily debated on television, though appeals to voters have largely been made on sectarian, ethnic or tribal grounds rather than the issues.
Attacks on polling stations and campaign gatherings in recent days have cast a pall over the vote, and spurred fears that much of the electorate could stay home rather than risk being targeted. In particular, the two days preceding the election saw more than 80 people killed in a spate of shootings and bombings, mostly concentrated in Baghdad and the restive north and west.
But many Iraqis have said they will head to voting centres regardless.
"I will go to vote... to take part in changing the current situation and the current faces after eight years of economic and security failure from a government in a country whose budget reaches tens of billions of dollars," said architect Ahmed Adel, 40, referring to the successive four-year terms secured by Maliki.
"We must go to the polls, whatever the circumstances. Whoever does not go is wasting his rights, and the rights of others."
More than 750 people have been killed this month, with unrest at its worst since Iraq emerged from a brutal sectarian conflict that left tens of thousands dead in 2006 and 2007.
Militants have controlled the town of Fallujah west of Baghdad -- the site of major battles during the insurgency against US-led forces -- since the beginning of the year. Parts of restive Anbar province, where Fallujah is located, will not be voting.