BAGHDAD, Dec. 27 -- The largest political party representing Iraq's Sunni Muslim minority announced Monday that it would drop out of the Jan. 30 election, dealing a fresh blow to the vote's credibility on the same day the top Shiite Muslim candidate survived a car bombing.

The withdrawal of the Iraqi Islamic Party, combined with the assassination attempt on cleric Abdul Aziz Hakim, heightened concerns that the parliamentary election may produce a lopsided result, further alienating Sunni areas where the armed insurgency is growing.

The need for adequate Sunni participation has become a central issue a month before the election, seen by the United States and Iraq's interim leadership as pivotal to creating a stable government. On Monday, Osama bin Laden, the al Qaeda leader, added his voice to those of Sunni clerics urging Iraqis to boycott the ballot, saying "anyone who takes part in this election consciously and willingly is an infidel."

Bin Laden issued the warning in an audiotape aired on the al-Jazeera satellite television network. In Washington, Secretary of State Colin L. Powell encouraged "all Sunnis and all Sunni leaders to join in this effort, to say no to terrorism, no to murder and yes to democracy."

But voter registration in Sunni areas has lagged far behind registration in other parts of Iraq, according to Iraq's top election official, Hussain Hindawi. Voters have not been able to register at all in Anbar province, home to the restive cities of Fallujah and Ramadi. Candidates have proved scarce as well: The 41 openings on Anbar's proposed provincial council have drawn only 50 candidates.

In another troubling sign, Western diplomats noted that preliminary indicators of voter participation nationwide are markedly lower than expected, judging by the sluggish early rate at which Iraqis have offered corrections to voter rolls.

Officials blame the problems on poor security and a late start in public information campaigns intended to explain the election to a population ruled by dictatorship for three decades. Leaders of the Iraqi Islamic Party also cited security as a reason for withdrawing the Sunni party's slate of 275 candidates.

"We asked to postpone the election long ago because we believe the security situation in the country is not suitable to hold elections," Mohsen Abdul Hamid, head of the party, told reporters in Baghdad.

"The Iraqis don't understand the elections yet," he said. "We need enough time, at least six months, to prepare ourselves. . . . The security issue is very complicated."

In the car bombing against Hakim, a vehicle driven by a suicide bomber exploded in rush hour traffic just after the Shiite cleric's motorcade entered a fortified compound on the Tigris River. At least 10 people were killed, including two guards and several motorists, according to U.S. military officials and one of Hakim's aides.

Hakim, who escaped injury, told the Reuters news agency that his group would not retaliate, noting that it had also not responded to the car bomb that killed his brother, Ayatollah Mohammad Bakir Hakim, in August 2003.

"The only ideology these people know is terror," Hakim said. "We laid down our arms in favor of pluralism. If we wanted violence, we would have responded a long time ago."

There was no indication as to who had carried out the attack.

Hakim heads the Supreme Council for the Islamic Revolution in Iraq. His name is first on the list of the Unified Iraqi Coalition, a slate of 228 candidates regarded as the leading contender for the votes of Shiites, who represent 60 percent or more of the population and whose political aspirations have been held in check by generations of Sunni leaders.

Hakim's party is also a major player in the interim government backed by the United States. At the same time, the cleric remains close to Iran, which for more than a decade sheltered and helped support his party and an allied militia.

Some Sunnis accuse Hakim's Supreme Council and another long-exiled Shiite party, Dawa, of rushing elections in order to cement their hold on power before challengers could can emerge. The Jan. 30 date was set during the U.S. occupation, with which both parties cooperated.

"The time which has been fixed by the United States will only fulfill the ambition of the two or three parties that are connected to Iran. It does not allow us to make real democracy in Iraq, so we want some extension that will let all Iraqis prepare," said Salih M. Mutlag, an academic and Sunni activist. Mutlag was among 600 delegates from six largely Sunni provinces who called for a postponement last week.

"If there will be an election Jan. 30, it will not be a celebration day as everybody wanted," Mutlag said. "It will be a doomsday in Iraq."

The public appetite for elections is unclear. In opinion polls taken earlier in the year, Sunnis voiced their intention to vote nearly as emphatically as did other Iraqis. And Hindawi, who heads Iraq's independent electoral commission, said Sunni tribal leaders and others had assured him that voters remain enthusiastic.

If Sunni voters have lagged in attending to voter lists, he said, "we are sure it's because of the security situation only. It's really not a political expression of the population."

That security situation includes chilling threats to would-be voters. A leaflet circulating this week in Diyala province northwest of Baghdad bears the headline: "Ultimatum Warning Threat" below a photo of former president Saddam Hussein. The flier vows that "our Jihadist battalions" will kill election organizers, blow up polling stations and "liquidate within 48 hours" anyone who votes.

Such threats may help account for the sluggish performance of Iraqis in the only task election officials have asked of them to complete so far: correct voter rolls. Election officials are using Iraq's food ration accounts as a master list for voter rolls. With last month's food ration, each household received a tally of residents recorded as age 18 or over. If the list was accurate, no action was required. If it wasn't, the head of household was asked to make a correction at a local election office.

Election officials expected as many as 2.8 million corrections, but the first weeks brought only about 200,000, according to diplomats briefed on the process.

Sadoun Dulame, a Sunni who heads the Iraq Center for Research and Strategic Studies, said turnout in Sunni areas is expected to be so low that the Iraqi Islamic Party withdrew rather than face defeat.

"In the end, it depends on the Sunni Triangle, and the Sunni Triangle doesn't vote in the election," he said.