The tone contrasted with the assertion in the "national strategy for victory in Iraq" unveiled by Bush on Wednesday that US forces were making "significant progress" in containing the insurgency.
Eisenstadt and White said the war in Iraq was still winnable, but added that the fight "will be protracted and costly, and is likely to be punctuated by additional setbacks."
US officials cited by the report estimated that the Sunni insurgency counted up to 20,000 members, including 3,500 active fighters. White said the total number of supporters could top 100,000.
While Washington has billed Iraq as the central front in its war on terror, White said foreign jihadists represented only 5-7 percent of the insurgency and did not account for the majority of attacks or fatalities.
But he said say the anti-US forces were making extensive use of religion and, in a new development, former members of Saddam's largely secular regime were identifying increasingly with the Islamists.
"There is some kind of merging going on," White told the lunchtime audience. "Whether this is a marriage of convenience or a marriage of commitment remains to be seen."
The report said the insurgency had no hierarchy, but was a "web of networks" drawing financial support from inside and outside Iraq. It said support from Syria and Iran was "not insignificant" but not essential.
"The insurgency has access to all the weapons, explosives, financial resources, and trained manpower it needs, in amounts sufficient to sustain current activity levels indefinitely -- assuming continued Sunni political support," it said.
The analysts said the insurgents had scored "important tactical and operational successes" while establishing themselves as a major force in the Sunni community and sowing doubts in the US about the continued presence of 160,000 US troops.
"This isn't just random activity or terrorist activity," White said. "The insurgents are actually conducting a purposeful kind of strategy in Iraq and are trying to counter the very kinds of things that we're trying to do."
But the report noted the insurgents were vulnerable on several counts, lacking a unified leadership and unqualified support from many Sunnis, and tarred by the brutality of the jihadists' attacks on civilians.
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