BAGHDAD, Iraq – Insurgents stepped up attacks yesterday against polling centers across Iraq, killing at least a dozen people, including an American Marine, in the rebel campaign to frighten Iraqis away from participating in this weekend’s election.

As part of an intensifying campaign of intimidation, an Al Qaeda affiliate led by Jordanian terror mastermind Abu Musab al-Zarqawi posted a videotape on the Internet showing the murder of a candidate from the party of interim Prime Minister Allawi.

The tape included a warning to Mr. Allawi personally: “You traitor, wait for the angel of death.”

To protect voters on Sunday, hundreds of American soldiers began moving out of their massive garrison on the western edge of Baghdad to take up positions at smaller bases throughout the city to respond more quickly to any election day attacks.

Sunni Muslim insurgents have threatened to disrupt the balloting, when Iraqis choose a 275-member National Assembly and governing councils in the country’s 18 provinces. Voters in the Kurdish self-governing area of the north will select a new regional Parliament.

In the former rebel stronghold of Fallujah, where opposition to the balloting is strong, American Marines drove through the city yesterday, urging people through loudspeakers to turn out Sunday. Spokesman 1st Lieutenant Lyle Gilbert said the Marines were “encouraging people to capitalize on this opportunity to exercise their voice by voting in the upcoming free elections.”

Iraqi newspapers also published for the first time the names of some 7,000 National Assembly candidates, many of whose identities had been kept secret to protect them from assassination.

The interim government will deploy an additional 2,500 troops to help guard the elections, the Defense Ministry said. A total of 300,000 Iraqi and multinational troops will provide security, with Iraq’s U.S.-trained forces taking the lead role.

About 9,000 Iraqi troops also are being dispatched to guard oil pipelines, which insurgents repeatedly have targeted.

Iraq’s national security adviser, Qassim Dawoud, acknowledged that security remains shaky in four provinces but expressed optimism that extensive security measures would protect voters and encourage turnout.

However, attacks were reported yesterday in at least seven provinces, including relatively peaceful Basra in the south, where terrorists fired mortar shells at four schools designated as polling stations.

American troops and rebels also exchanged fire yesterday on Haifa Street in central Baghdad, witnesses said.

One Marine was killed and five others were wounded when insurgents fired mortars at their base near Iskandariyah, about 30 miles south of Baghdad in tense Babil province.

Another three Iraqis were killed and seven injured when a roadside bomb missed an American convoy in Mahmoudiya, a religiously mixed area of Babil province, hospital officials said.

Most of the attacks occurred in Salaheddin province in an area of the Sunni triangle north and west of Baghdad that American and Iraqi officials have identified as one of the key trouble spots. Three Iraqi civilians were killed yesterday when a car bomb exploded in Samarra, 60 miles north of Baghdad.

Hours later, mortar shells fell on a designated polling station in Samarra, police said. Armed men in Samarra blew up a school administration building after first ordering the staff to leave, police Lieutenant Qassim Mohammed said. The destroyed building had been scheduled to be a voting center Sunday.

Sporadic clashes also erupted in Samarra between American troops and armed men, killing one Iraqi civilian and injuring another, Mohammed said. A suicide car bomber struck an American military convoy near the Salaheddin provincial city of Beiji, witnesses said. No casualties were reported.

A roadside bomb targeting an American convoy killed one Iraqi bystander near the Salaheddin provincial capital of Tikrit, police said.

In Tamim province, insurgents attacked seven polling stations in the city of Kirkuk with mortars and machine guns, killing one policeman, authorities said.

An Iraqi army soldier was killed and seven people were injured when a suicide car bomb exploded near an Iraqi patrol in Baqouba in Diyala province, American officials said.

In Ramadi, capital of the insurgent plagued province of Anbar, an Iraqi national guard soldier was killed when insurgents attacked a joint American-Iraqi force guarding a voting center at a school, police Lieutenant Safa al-Obeidi said.

An American soldier died yesterday from a gunshot wound on a base near Tikrit in what the American military command said was an accident.

American and Iraqi forces have stepped up security operations throughout the country ahead of the vote. Iraqi army chief General Babaker Shawkat Zebari said that in the past three weeks Iraqi authorities have detained 2,000 suspected insurgents, including foreigners from Egypt, Saudi Arabia, Syria, and Yemen.

Some of those arrested have been identified as members of Mr. al-Zarqawi’s terror network, responsible for numerous car-bombings, kidnappings and beheadings of foreign hostages.

In the videotape, Mr. al-Zarqawi’s group showed a man identifying himself as Salem Jaafar Abed, a National Assembly candidate on Mr. Allawi’s ticket. “I advise all my brothers, especially the youth to stay away from supporting and cooperating with the occupying enemy,” the man said on the tape before he was shot. “I’m ready to work to serve Iraq and the people of Iraq,” he added.

Also yesterday, President Bush said in an interview with the New York Times that he would withdraw from Iraq if the government that is elected on Sunday asks him to. He added that he anticipated the leader’s wanting American troops to remain in the country “at least until the Iraqis are able to fight.”