The shooting of Ayid Abd al-Ghani occurred in new Baghdad, an eastern section of the capital, at about 7.45am (0445 GMT) on Tuesday, said police Major Falah al-Muhammadawi.
The attackers either drove up beside Abd al-Ghani's vehicle and opened fire or first stopped the victim's car by blocking it, al-Muhammadawi said.
Abd al-Ghani was an adviser to Usama al-Najafi, Iraq's industry minister and one of the country's top Sunni Arabs.
Before Iraq's constitutional referendum on Saturday, al-Najafi had predicted that voters would reject the document if given the chance to vote freely because it favours Kurds and the Shia over the Sunni Arabs.
Vote counting was still under way on Tuesday, and the outcome of the referendum was not expected to be announced by Iraq's election commission for several days.
US marines killed
Two US marines have been killed in fighting near the town of Ar-Rutbah in western Iraq's rebellious Al-Anbar province, the military said Tuesday.
The two "were killed in action by small-arms fire while conducting combat operations against the enemy in the vicinity of Ar-Rutbah (on Monday)", it said in a statement.
US and Iraqi forces have been waging offensives in the Sunni-dominated province for several months.
On Sunday, the military said air strikes by US helicopters and combat jets killed 88 rebels in Al-Anbar, although television reports and a hospital source in Ramadi said some of the victims were civilians.
No quick end seen
On Monday, former US secretary of state Colin Powell said the constitutional referendum would not bring a quick end to the fighting.
Colin Powell speaks to talk show
host Larry King on Tuesday
"This is an important step forward, but we should not think that this is the end of the game," Powell said on CNN's Larry King Live.
The United States hopes the constitution will help stabilise Iraq and ultimately lead to the withdrawal of US forces.
But Powell said the United States still faced a "difficult" and "sustained insurgency" that needed to be beaten on the ground as well as politically.
"We are still in for some difficult days ahead, but we needed to get this referendum dealt with now so we can move on to find a permanent government through an election at the end of the year," Powell said.
Powell, a retired four-star general who was secretary of state at the start of the US-led invasion of Iraq, said he was surprised at how robust the fighters became so quickly after the fall of Baghdad.
"Frankly, I think we should have done more in the beginning to push that to the back, knock it down before it got started," he said.