Here are profiles of five key players:
- Nuri al-Maliki -
The tough-talking prime minister is standing for a third term as his adminstration struggles to contain the rising violence.
Maliki spent decades in exile but returned after the US-led invasion and fro 2006 headed Iraq's first permanent government after Saddam Hussein.
The 63-year-old rarely smiling Shiite forged a reputation as a strong leader who imposed some stability after coming to power.
In the 2010 election, his State of Law Alliance came second to a Sunni-backed cross-sectarian grouping but he joined forces with a coalition of other Shiite parties to form the foundation of a national unity government.
The past year's rise in violence, the worst in years, has dented his credibility.
Maliki has also been criticised for allegedly authoritarian tendencies, although he insists he is trying to manage an unruly coalition.
- Osama al-Nujaifi -
The speaker of parliament and Iraq's most senior politician from the Sunni Arab minority.
Once a member of the secular, Sunni-backed Iraqiya bloc that won the most seats in 2010 and long at odds with Maliki, Nujaifi has since formed his own party which he leads with his brother, the governor of the northern province of Nineveh.
When troops moved against Sunni anti-government protesters near the northern Sunni Arab town of Hawijah in April last year, Nujaifi called on the cabinet to resign and for early elections in the name of "national reconciliation".
He was unhurt when his convoy was attacked in his home town, the northern city of Mosul, in February.
- Ali al-Sistani -
Elderly Shiite cleric Ayatollah Ali al-Sistani stays generally aloof of politics, but is adored by millions and is highly influential.
Sistani is the leading member of the "marjaiya", the top Shiite council in Iraq, and is more respected than any Shiite politician. He has made occasional political interventions since 2003, calling for a massive voter turnout in 2010 but without endorsing any party.
His return to Najaf in August 2004 from medical treatment in London settled a deadly confrontation between American troops and the Shiite Mahdi Army militia.
Sistani also pressured Washington to expedite the path to democratic elections in early 2004, and was the guiding force behind the creation of the pan-Shiite coalition in parliament from 2006 onwards.
He repeatedly called for calm during the brutal sectarian conflict from 2006 to 2008.
- Moqtada al-Sadr -
The Shiite cleric who heads a once-feared militia said in February he was bowing out of politics, but remains a potential kingmaker.
With his grey-streaked, bushy black beard and black turban of a "sayyid" or descendant of the Prophet Mohammed, Sadr gained widespread popularity after the 2003 invasion.
After backing Maliki in 2006, ensuring that he became premier, Sadr in 2007 ordered his followers to withdraw from the cabinet, almost bringing down the government.
Sadr again supported Maliki when the latter finally formed a post-election government in December 2010, but has since been a sharp critic.
After withdrawing from politics, Sadr gave a speech calling Maliki a "tyrant".
- Massud Barzani -
The president of Iraq's autonomous Kurdish region. Barzani and his Kurdistan Democratic Party have long formed a duopoly with Iraq's President Jalal Talabani, until Talabani's Patriotic Union of Kurdistan faltered in recent regional elections.
The son of Kurdish nationalist leader Mullah Mustafa Barzani, he became KDP leader in 1979.
After the 2003 invasion, Barzani and Talabani agreed that Talabani would become overall president and Barzani would be president of the Kurdistan Regional Government.
Though he brokered the deal that saw Maliki retain the premiership in 2010, Barzani is now a vocal opponent of the prime minister.