Insurgents exposed the vulnerability of Iraq's security forces again Sunday, killing at least 22 National Guardsmen and their driver in a car 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 U.S. military sent new forces to counter the threat in Mosul (search), center of a worrying rise in car bombings and raids in recent weeks.

Secretary of State Colin Powell (search) later repeated past warnings of more violence ahead of the Jan. 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 far more casualties.

The worst attack Sunday occurred north of Baghdad when a bomber detonated his explosives-laden vehicle next to a bus carrying Iraqi National Guard (search) troops. Police Lt. Haidar Karar in Balad, where the attack occurred, said 18 guardsmen and the driver were killed in the initial blast and four more guardsmen died of their wounds later.

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 a U.S. base in Balad, 50 miles north of Baghdad, U.S. military spokesman Maj. Neal E. O'Brien said. Balad is in the so-called Sunni Triangle, the scene of frequent assaults on U.S. and Iraqi security forces.

"Those responsible for suicide attacks are seeking to halt Iraq's progress on the path to democracy," O'Brien said.

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

The Shiite leaders who spoke Sunday belong to the Unified Iraqi Alliance, a mainstream Shiite coalition running in the election. The group is 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, have repeatedly targeted Shiites in apparent attempts to widen sectarian rifts.

"The Iraqi Unified Alliance calls for a national talks to stand against the civil war or sectarianism conflict," said Sheikh 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 percent 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 percent 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 Usama bin Laden in a tape released 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 U.S.-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," al-Hakim said. "The whole world and neighboring countries and other countries have to know that the Iraqi people have chosen its path."

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.

Chalabi's spokesman, Entifadh Qanbar, said 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.

Qanbar said Chalabi told Iranian leaders the list "is an Iraqi list and this means that there will not be an Islamic state" in Iraq.

One of the worst trouble spots in recent weeks, however, has been the northern city of Mosul, which saw insurgents rise up and overwhelm several police stations in November as well as kill more than 100 National Guard troops.

On Sunday, military spokesman Lt. Col. Paul Hastings said two brigade-sized units, consisting of Iraqi forces and elements of the 82nd Airborne Division, were deployed to augment the 8,000 troops already in Mosul. Brigades can include 2,000-4,000 soldiers.

Also Sunday, a car bomb struck a U.S. Task Force Baghdad patrol southwest of the capital, wounding two soldiers, the military said.

Several other slayings were reported Sunday. Four policemen were killed while on patrol in Samarra, 60 miles north of Baghdad, the military said, while gunmen shot dead a deputy governor of the eastern Diyala province, hospital officials reported.

Also, the police chief in Jebala, a town 40 miles south of Baghdad, was killed when gunmen stormed his house, police said. Assailants killed a police officer in Basra, and police officer and a Muslim cleric in separate Baghdad attacks, officials said.

Meanwhile, the government issued a statement Sunday saying Iraqi National Guards detained 217 suspected insurgents in Mahmoudiyah, a town about 25 miles south of Baghdad.