BAGHDAD, Iraq, July 23 - They just keep getting stronger.

Despite months of assurances that their forces were on the wane, the guerrillas and terrorists battling the American-backed enterprise here appear to be growing more violent, more resilient and more sophisticated than ever.

A string of recent attacks, including the execution of moderate Sunni leaders and the kidnapping of foreign diplomats, has brought home for many Iraqis that the democratic process that has been unfolding since the Americans restored Iraqi sovereignty in June 2004 has failed to isolate the insurgents and, indeed, has become the target itself.

After concentrating their efforts for two and a half years on driving out the 138,000-plus American troops, the insurgents appear to be shifting their focus to the political and sectarian polarization of the country - apparently hoping to ignite a civil war - and to the isolation of the Iraqi government abroad.

And the insurgents are choosing their targets with greater precision, and executing and dramatizing their attacks with more sophistication than they have in the past.

American commanders say the number of attacks against American and Iraqi forces has held steady over the last year, averaging about 65 a day.

But the Americans concede the growing sophistication of insurgent attacks and the insurgents' ability to replenish their ranks as fast as they are killed.

"We are capturing or killing a lot of insurgents," said a senior Army intelligence officer, who spoke on condition of anonymity because he was not authorized to make his assessments public. "But they're being replaced quicker than we can interdict their operations. There is always another insurgent ready to step up and take charge."

At the same time, the Americans acknowledge that they are no closer to understanding the inner workings of the insurgency or stemming the flow of foreign fighters, who are believed to be conducting a vast majority of suicide attacks. The insurgency, believed to be an unlikely mix of Baath Party die-hards and Islamic militants, has largely eluded the understanding of American intelligence officers since the fall of Saddam Hussein's government 27 months ago.

The danger is that the violence could overwhelm the intensive American-backed efforts now under way to draw Iraq's Sunni Arabs into the political mainstream, leaving the community more embittered than ever and setting the stage for even more violence and possibly civil war.

Fakhri al-Qaisi, a conservative Sunni leader, warned that if the isolation of Iraq's Sunnis was not soon reversed, the insurgents would grow even stronger.

"They will make suicide bombs, and they will destroy all," Mr. Qaisi said.

Such results appear to be exactly what the insurgents are trying to bring about.

On Tuesday, masked insurgents gunned down two moderate Sunni leaders who had been helping to draft Iraq's permanent constitution. The killings, carried out in the middle of a busy Baghdad street in heavy traffic, appeared to be calculated to squelch the voices of moderate Sunnis, and to prevent anyone else from stepping forward.

The immediate effect seemed to play right into the insurgents' hands: moderate Sunni leaders announced that they were suspending their efforts to help draft a constitution, laying down several conditions for their return.

Insurgents have killed moderate Sunni leaders before, but the shootings of Mejbil al-Sheik Isa and Damin al-Obeidi on Tuesday were especially striking: the men were killed after months of coaxing by Iraqi Shiite leaders and American officials intended to bring moderate Sunnis like them into the constitutional process.

The killing of the Sunni leaders came just three days after one of the worst suicide attacks since the American invasion, and one that was clearly intended to draw the country closer to a full-blown sectarian conflict.

Last Sunday, in the Shiite town of Musayyib, about 40 miles south of Baghdad, a suicide bomber dashed beneath a truck full of liquefied gas and blew himself up, igniting a giant fireball that killed more than 70 people and wounded at least 156. The truck, which amounted to a gigantic bomb itself, had been hijacked in western Iraq and parked next to a Shiite mosque.

The deadliness of the attack, and its obvious sectarian intent, prompted unusual expressions of alarm from Iraq's Shiite leadership, which had until now spoken with confidence of Iraq's ability to avoid sectarian strife. Grand Ayatollah Ali al-Sistani, the country's most powerful Shiite leader, called on the Shiite-dominated government "to defend this country against mass annihilation."

Then, on Thursday, the rebels struck again, kidnapping the top Algerian diplomat in Iraq and a colleague. The gunmen snatched Ali Billaroussi, the top envoy, and Azzedine Belkadi, in Mansour, one of Baghdad's best neighborhoods, in broad daylight.

The abduction of the two diplomats followed the kidnapping and killing earlier this month of Ihab al-Sharif, Egypt's top diplomat, who had been designated to become the Arab world's first ambassador to Iraq. The kidnappings seemed designed to intimidate foreign governments, particularly Muslim governments, into withholding full diplomatic relations with the fledging Iraqi government.

As with the slaying of the moderate Sunni leaders, the kidnappings have seemed, so far, to have secured exactly what the insurgents wanted. No Arab government has yet sent an ambassador to this country.

In Baghdad, it is commonly understood that the recent success of the insurgency lies in part in the weakness of the Iraqi government. The Sunni leaders who were slain, for instance, were traveling with a single guard, whom one of the Sunni leaders had provided at his own expense. Pleas by the two Sunni leaders to the Iraqi government for protection had apparently gone unheeded.

And in the case of the bombing in Musayyib, Iraqi officials said the gas truck, owned by the Oil Ministry, had been hijacked by insurgents on its way from Baghdad to Falluja several days before the bombing. To get to Musayyib, the truck probably passed through numerous military and police checkpoints, yet somehow, it reached its destination.

On Saturday, the police said a Sunni Arab from a village near Falluja admitted that he was part of a group that carried out the bombing, and confirmed that it had hijacked the gas truck and sent the suicide bomber. The man was arrested after a shootout in which two other suspects were killed.

Still, part of the explanation for the insurgents' resiliency stems from their own shrewdness. American commanders believe that the rash of diplomat kidnappings came after the Jordanian militant Abu Musab al-Zarqawi formed a cell in Baghdad specifically for abducting diplomats.

One other recent development in the insurgency - and a possible explanation of its ability to bring in recruits from around the Arab world - is the reach and sophistication of its public relations.

Most of the main insurgent groups - like Al Qaeda in Mesopotamia and Ansar al Sunna - regularly post updates of their exploits on the Web. Scarcely a day passes when one of the groups has not announced another attack with either video or printed notice.

A communiqué released Friday by Ansar al Sunna, for instance, boasted of an attack on an American Humvee in the northern Iraqi city of Samarra. The Humvee, the communiqué said, had been destroyed with an "explosive package."

"Four crusaders who were in it were killed," the notice said.

"God is great," the notice concluded in the usual way. "Glory to God, his Messenger, and to the believers."

Eric Schmitt contributed reporting from Washington for this article, and John F. Burns from Baghdad.