The cleric, Sheik Ghalib Ali al-Zuhairi, was killed as he left a mosque after dawn prayers in the town of Muqdadiyah, 60 miles north of Baghdad, police said.
His assassination occurred a day after another prominent Sunni cleric was killed in the northern city of Mosul - Sheik Faidh Mohamed Amin al-Faidhi, who was the brother of the association's spokesman. It was unclear whether the two attacks were related.
Insurgents hit a U.S. convoy with a roadside bomb near the central Iraq city of Samarra, prompting the Americans to open fire, killing an Iraqi, hospital officials said. Mortar rounds aimed at a nearby U.S. military base injured two children.
The joint military operation kicked off with early morning raids in the town of Jabella, 50 miles south of Baghdad, as Iraqi and American troops, backed by jets and helicopters, swarmed into the region known as the "triangle of death."
At least 32 suspected insurgents were captured in the morning's raids, the U.S. military said. In other joint raids conducted in Iskandariyah and Latifiyah, another 45 suspected terrorists were arrested, said Iraqi police Capt. Hadi Hatif.
Britain's 1st Battalion of the Black Watch Regiment, which was brought to the area from the southern city of Basra to aid U.S. forces, was also involved in closing off militant escape routes between Baghdad, Babil province to the south and Anbar province to the west.
"We believe some fighters from Fallujah skirted away and came down to our area to, among other reasons, take a little bit of pressure off of Fallujah," said Capt. David Nevers, a spokesman for the 24th Marine Expeditionary Unit.
Another reason for the increase in attacks might also be that Iraqi and Marines stepped up house-to-house searches and vehicle checkpoints in the area for the last three months, detaining nearly 250 insurgents, he said.
"For the last couple months, we've gone into areas that had formerly not seen a lot of presence ... We went in and stirred up a few hornets nests," Nevers said.
The assault follows the massive offensive against Fallujah, in which at least 54 U.S. troops were killed and 425 wounded.
In the wake of that operation, insurgent attacks throughout central and northern Iraq stepped up dramatically. Earlier this month, northern Mosul witnessed a mass insurgent uprising, and some 2,400 U.S. and Iraqi troops were sent in to retake control.
Since the Fallujah offensive began Nov. 8, some 850 U.S. servicemembers have been wounded throughout Iraq, bringing the total for the entire war over 9,000, the Pentagon said Tuesday.
The new offensive is aimed at stemming the wellspring of violence that has engulfed much of the country ahead of the elections. But the recent attacks against the Sunni clerics, as well as last week's deadly U.S. and Iraqi raid on Baghdad's Abu Hanifa mosque, raise troubling questions about whether the elections can unify the religious and ethnic divisions in Iraq.
The Association of Muslim Clerics has called for a boycott of the vote, and if many Sunni Arabs refuse to vote, it could undermine the election.
Al-Faidhi, the 41-year-old cleric killed in Mosul on Monday, was the head of a religious school and a popular figure who was well-liked by the Shiite and Kurdish communities in Mosul.
"He was against the American occupation to Iraq but he opposed the use of violence, preferring peaceful means and politics," said family member Mohammed Khadr. "His goal is to unite the Muslims around the world. He insisted on making Kurds part of the community in Mosul and he managed to do that."
Mohammed Bashar al-Faidhi, the slain cleric's brother and an association spokesman in Baghdad, said he believes the Israeli intelligence agency Mossad was behind the assassination of his brother, along with "some Iraqi elements."
Meanwhile, a top aide to radical Shiite cleric Muqtada al-Sadr accused the government of violating terms of the August agreement that ended an uprising by al-Sadr's followers in Najaf.
Ali Smeisim, al-Sadr's top political adviser, made no explicit threats but his remarks raised the possibility of a new confrontation with al-Sadr's Mahdi Army militia, which fought heavy battles against the Americans and their Iraqi allies in April and August.
Smeisim accused the government of breaking an agreement not to arrest members of al-Sadr's movement and to release most of those being held in detention. "Iraqi police arrested 160 al-Sadr loyalists in Najaf four days ago," he said.