Gunmen assassinated a high- profile Sunni Muslim politician on Tuesday, a stark reminder of dire insecurity two days before Iraqis vote for their first full-term parliament since Saddam Hussein's fall.
Mizhar al-Dulaimi, who ran his own political party, was shot dead as he campaigned in Ramadi, a violent city west of Baghdad, police said. Three of his bodyguards were wounded.
He was the latest of several influential Sunni Muslims, including a top cleric, to be killed ahead of Thursday's poll, as militants try to sabotage the US-backed political process.
The US ambassador to Iraq, Zalmay Khalilzad, condemned Dulaimi's murder and urged Iraqis to go ahead and vote on Thursday.
"This is a defining moment," Khalilzad told reporters. "While I encourage participation from all, the United States does not endorse any candidate."
Dulaimi's killing came a day after the first votes were cast in the election - detainees, Iraqi security forces and hospital patients all voted on Monday - and as up to two million Iraqis living abroad also began to cast their ballots.
More than 15 million Iraqis are registered to vote in what the poll's supporters hope will be a watershed event, ushering in a four-year, 275-seat parliament and a new government to tackle rampant violence as foreign forces begin to withdraw.
Security for the election will be stringent with Iraq's borders and airspace closed and travel between provinces banned. More than 150,000 Iraqi police and soldiers will ring 6,000 polling sites and the next five days are a national holiday.
Campaigning officially ends on Tuesday, leaving a day of reflection for voters ahead of the poll. Aside from Dulaimi's assassination, the country was remarkably calm on Tuesday.
Militant groups have told Iraqis not to go to the polls, calling the election a "Satanic project", but most have steered away from the grim threats to kill all voters that they made ahead of the last election in January.
The poll is expected to see far higher participation than last time among Iraq's Sunni Arab minority, who form the backbone of the two-year insurgency against the government and US forces. The Sunnis largely boycotted January's poll.
That boycott is now seen as a mistake. Sunnis, powerful under Saddam, saw their influence greatly undermined and Sunni leaders are now determined to make amends for the error.
Several Sunni Arab lists are registered for the poll and are expected to pick up strong support in central and western Iraq, even if some elements remain adamantly opposed to the process.
US President George W. Bush offered encouragement to Iraqi voters ahead of the ballot in a speech on Monday, but also acknowledged that the election would not be perfect.
"A successful vote is not the end of the process. Iraqis still have more difficult work ahead," he said. "These enemies aren't going to give up because of a successful election."
Tuesday is the 1,000th day since US-led forces invaded to overthrow Saddam, and the second anniversary of his capture.
The Bush administration hopes the election will cement a transition to democracy and lay the groundwork for some of the 170,000 US and other foreign troops to withdraw.
Whatever government emerges will have as its top priority the bolstering of Iraqi security forces to take on the insurgency alone - the central platform of Bush's plan for a staggered pullback of his own troops.
But there will also be complex domestic and social problems to face, including delivering electricity and clean water, making the oil sector work, renegotiating parts of the constitution and deciding how best to handle de-Baathification, the removal of members of Saddam's Baath party from public life.
A poll commissioned by the BBC ahead of the vote showed that turnout could be as high as 80 percent, up from just under 60 percent in January's election, itself a strong showing.
The campaign has been more sophisticated than in January, with candidates running slick adverts on television and wooing the electorate via mobile phone text messages.