But the group is increasingly breaking ranks over government policies it sees as ignoring Sunni concerns.
Last month, the party withdrew from the government of interim Prime Minister Iyad Allawi in protest over the U.S.-led military operation to retake Al-Fallujah. At the time, the party cited concerns over civilian deaths and worries that "our presence in the government will be judged by history" unfavorably.
Now, the party says it will not take part in nationwide elections on 30 January to choose a new National Assembly. The assembly is to select Iraq's next interim government and appoint the body that will write Iraq's first post-Saddam Hussein constitution.
Yesterday, the head of the Iraqi Islamic Party, Muhsin Abd al-Hamid, said the party is withdrawing because conditions are not right for a poll. But he said his party will not call on Iraqis to boycott the vote -- as some Sunni community leaders have done.
"We announced our withdrawal and not a boycott [of the elections]. We already announced we would take part in these elections under certain conditions. These conditions were not met. The elections will not be 'general' until they cover all parts of Iraq," al-Hamid said.
The Iraqi Islamic Party had asked the government to postpone the election for at least six months until the security situation in the country improves. Past weeks have seen a rise in insurgent attacks intended to disrupt preparations for the poll, particularly in Sunni-majority central Iraq.
Mustafa Alani is a regional specialist at the Gulf Research Center in Dubai. He said the major Sunni party does not want to give legitimacy to a poll in which it feels security problems will virtually ensure it does poorly. "They believe that the Arab Sunnis' area is not secure," he said. "And even if they are going to participate, giving this sort of process legitimacy as an Arab Sunnis' party, they are going to lose a huge number of votes because of the security problem."
Alani said the party's withdrawal could make it unlikely Sunnis will turn out in large numbers for the poll. He said the move now leaves the field in central Iraq mainly to individual Sunni candidates, who lack the resources to mobilize voters or to resist threats from insurgents.
A variety of extremist groups have made public threats in recent days against anyone who runs for the assembly or votes in the election.
An audiotape attributed to Osama bin Laden and broadcast yesterday by the Qatar-based Al-Jazeera satellite station said that any Iraqi who votes on 30 January will become a "nonbeliever." The tape expresses support for the extremist group led by Abu Mus'ab al-Zarqawi, which has often taken credit for suicide bombings against U.S. and Iraqi government targets that have also killed civilians.
Leaflets circulated in Diyala Province by unknown groups this week vow that "Jihadist battalions" will kill anyone "within 48 hours" of voting.
In Washington, U.S. Secretary of State Colin Powell called on Sunnis to ignore all election threats. He said he encourages "all Sunnis and all Sunni leaders to join in this effort, to say 'no' to terrorism, 'no' to murder, and 'yes' to democracy."
Many analysts say it is too early to predict to what extent the Sunni community will participate in the poll. Prime Minister Allawi sought during talks this month in Amman, Jordan, to enlist Iraqi Sunni tribal and other leaders in bringing out the vote. It is not known to what extent his efforts succeeded.
But analyst Alani said there are signs that the Sunni community increasingly views elections at this time as harmful to its interests. He noted that there is widespread fear among Sunnis that the vote will hand power to the Shi'a majority and suspicion that neighboring Shi'a Iran is interfering in Iraq to make that happen:
"At the level of many ministers of the [Iraqi] government, they clearly are talking about Iranian intervention in the election. So [the Sunnis] think that given all these difficulties and the fact that there is an external intervention in this -- or potential intervention -- I don't think there is a possibility that they will participate," Alani said.
Allegations of Iranian intervention include charges that Tehran has sent up to 1 million Iranians into Iraq to register to vote, as well as funneled large amounts of money to Shi'a parties. Tehran denies it is interfering in Iraq's domestic affairs.
Both Iraq's security problems and the potential for heightened communal tensions over the poll were highlighted by an apparent assassination attempt in Baghdad yesterday against the leader of the best organized Shi'a party, the Supreme Council for the Islamic Revolution in Iraq (SCIRI).
In the attack, a vehicle driven by a suicide bomber exploded just after a motorcade carrying SCIRI head Abd al-Aziz al-Hakim entered the party's fortified headquarters. Al-Hakim was unhurt, but at least 13 people outside the compound were killed.
There has been no indication of who carried out the attack, and al-Hakim said his party will not respond to it. The SCIRI leader heads the list of one of the leading slates of candidates that will vie for votes on 30 January, the Unified Iraqi Coalition.
The slate, endorsed by preeminent Shi'a leader Grand Ayatollah Ali al-Sistani, is expected to garner a large portion of the vote within the Shi'a community, which makes up some 60 percent of the Iraqi population.
[For the latest news on Iraq, see RFE/RL's webpage on "The New Iraq".]