BAGHDAD, Iraq – Insurgents exposed the vulnerability of Iraq’s security forces again yesterday, killing at least 22 national guardsmen and their driver in a suicide bombing, and 10 other people in separate attacks with elections just weeks away. Prominent Shiite leaders called for unity with Sunni Arabs wanting to delay the vote but insisted it be held despite the violence.

Also, the American military sent new forces to counter the threat in Mosul, center of a worrying rise in car bombings and raids in recent weeks.

Secretary of State Powell later repeated past warnings of more violence ahead of the January 30 elections for a national assembly, and the guerrillas have made good on those fears with tragic ease. Iraq’s poorly equipped security forces usually have far less training than American troops, and attacks on them usually result in more casualties.

The worst attack yesterday occurred in Balad, 50 miles north of Baghdad, when a suicide bomber detonated his explosives-laden vehicle next to a bus carrying Iraqi national guard troops. Police Lieutenant Haidar Karar said 18 guardsmen and the driver were killed in the initial blast and four more guardsmen later died of their wounds.

American officials said they were still trying to determine if there were one or two attackers inside the car that detonated next to the bus.

It was the deadliest assault on Iraqi security forces since October, when insurgents gunned down about 50 new national Guardsmen at a fake checkpoint. Another national guardsman was killed separately, south of Kirkuk, officials said. The car bomb detonated as the bus passed an American base in Balad, American military spokesman Major Neal O’Brien said. Balad is in the so-called Sunni Triangle, the scene of frequent assaults on American and Iraqi security forces.

The Shiite leaders who spoke yesterday belong to the Unified Iraqi Alliance, a mainstream Shiite coalition running in the election. The group was expected to do extremely well in the election and its leaders likely will have top government posts if the vote goes through.

They reached out to Sunni Arabs, many of whom are boycotting the vote and have sought a delay, and called for talks to avert civil war. Iraq’s insurgents, believed to be predominantly Sunni, repeatedly have targeted Shiites in apparent attempts to widen sectarian rifts.

“The Iraqi Unified Alliance calls for national talks to stand against the civil war or sectarianism conflict,” said Sheik Humam Hamoudy, a member of the Supreme Council for the Islamic Revolution in Iraq, which is part of the coalition. “We call for unity particularly with the Sunni brothers because there is a large plan to create a sectarian fight.”

Shiites, who make up about 60% of Iraq’s 26 million people, are eager for the vote to go ahead so they can take power long denied them when the Sunni Arab minority had power under Saddam Hussein. But they hope the Sunnis, who make up about 20% of the people, will participate lest the vote be considered illegitimate.

The Shiite leaders, who are backed by Iraq’s most influential cleric, Grand Ayatollah Ali al-Sistani, said postponing the vote would only create more chaos in Iraq. They rejected comments purportedly made by Osama bin Laden in a tape released last Monday in which the Al Qaeda leader urged Muslims not to vote, calling the election illegitimate.

“We believe in the joint participation of all the components of the Iraqi people,” said Abdul Aziz al-Hakim, head of Sciri. “For us the elections are vital and we will not give them up. Bin Laden is interfering in the Iraqi affairs by calling his criminal followers to hinder Iraqis from voting.”

Ahmad Chalabi, a former exile and one-time Pentagon confidant who led the Iraqi National Congress, said that while no Iraqi wants American-led coalition forces to remain in Iraq, the alliance would not seek the troops’ immediate withdrawal after the vote.

While many of the parties in the alliance are believed backed by neighboring Iran, the leaders repeated their theme that no other country should interfere in Iraq’s affairs – a comment also directed at Jordan, whose King Abdullah told the Washington Post recently that more than 1 million Iranians have entered Iraq, many to vote, and were being encouraged by the Iranian government.

“Iraqis do not want others, whoever they are, to interfere in the Iraqi affairs,” Mr. al-Hakim said.

According to the laws for the ballot, Iraqis will choose between candidate lists, and each coalition will be represented based on the percentage of the vote they receive. Individual candidates also may run.

Mr. Chalabi’s spokesman, Entifadh Qanbar, said Mr. Chalabi was recently in Iran and told officials there the new leaders would not establish a theocracy like the Tehran government, as some have feared.