BAGHDAD, Iraq – A suicide bomber wearing a police uniform blew up his car at police headquarters in Tikrit, killing at least 15 people in Saddam Hussein’s hometown in the bloodiest of several attacks yesterday that claimed 30 lives. Two American soldiers were among the dead.

The suicide bombings and other attacks came as politicians negotiated behind the scenes to forge the alliances needed to win enough backing in the 275-seat National Assembly for the post of prime minister.

The American command said two American soldiers were killed and two wounded in separate bomb attacks: one northeast of Baghdad in Qaryat, and a second near Samarra, west of Qaryat.

In the Sunni Arab stronghold of Tikrit, 80 miles north of Baghdad, a man dressed as a police lieutenant drove through the station’s gates and blew himself up just as dozens of policemen were arriving to relieve colleagues who had worked through the night, police Colonel Saad Daham said.

“He waited until the shift change, then he exploded the car,” Colonel Daham said, adding that the aim was “to kill as many as possible.” At least 22 people were wounded.

Twenty cars were set ablaze after by the blast, sending up clouds of smoke. An Associated Press photographer saw at least 10 charred bodies on the ground, which was splattered with pools of blood.

A suicide bomber killed five other people in Iskandariyah, 30 miles south of the capital, when he blew himself up in front of the local headquarters of a key Shiite alliance member, the Shiite Supreme Council for the Islamic Revolution in Iraq.

Police initially said the attack targeted the police chief, Colonel Salman Ali, who escaped unharmed.

In Baghdad, gunmen fired on a bakery, killing two people and wounding a third, police said. Several blasts echoed through the capital at midday and several more after nightfall. Their causes were not known.

Two roadside bombs in Qaim, near the Syrian border, killed four Iraqi National Guardsmen, Iraqi Lieutenant Colonel Abid Ajab Al-Salmani said.

Elsewhere, insurgents ambushed a police patrol in the northern city of Kirkuk with a roadside bomb, killing two policemen and injuring three.

American Marines and Iraqi troops, meanwhile, pressed a joint operation to root out insurgents in parts of the so-called Sunni triangle. The military said it detained 17 suspected insurgents and seized several weapons caches.

Politicians of all stripes sought out representatives of Iraq’s Sunni minority, whose support they need to isolate the insurgency. Many insurgents are thought to be loyalists of Saddam’s outlawed Sunni-dominated Baath Party.

A powerful Sunni organization believed to have ties with the insurgents, the Association of Muslim Scholars, rejected any role in the government – or even in writing a new constitution. Another Sunni group, the Iraqi Islamic Party, also rejected offers but agreed to help with the constitution.

The dominant United Iraqi Alliance, which nominated Ibrahim Jafari, leader of the Islamic Dawa Party, for prime minister, claimed yesterday that it won the support of eight members of three tiny parties and boosted its parliamentary strength to 148.

Alliance member Salama Khafaji said the groups were the Iraqi Turkoman Front, the National Independent Elites, and the Islamic Labor Movement in Iraq. But a splinter group thought to represent about 30 seats in the alliance, and which once supported one-time Bush administration favorite Ahmad Chalabi, renewed threats to withdraw its support. Although they issued no demands, it was unclear what Mr. Chalabi – who withdrew from the race – had promised them for their support.

Prime Minister Allawi, the secular Shiite who has about 40 seats, tried to take advantage of the rift by attempting to open talks with the Shiite splinter group just one day after announcing he would form a broad coalition to try to hold onto his post.

Two days after Dr. Jafari was nominated by the clergy-backed United Iraqi Alliance, the Shiite Political Council once allied with Mr. Chalabi again threatened to withdraw. The group claims to control 30 members of the National Assembly.

Ali Faisal, the council’s political coordinator, said his group received two phone calls from Qassem Dawoud, a senior Allawi party member and interim national security adviser. He said Mr. Allawi was trying to win their support.