The analysts say that only if U.S. designation of al-Nusra were followed by serious weapons supplies to the "good" rebels might those rebels change their stance toward the group.
While President Barack Obama has not ruled out the option of supplying weapons, officials say there first must be more clarity on the ground.
"For us, providing arms has to be done in a way that helps promote a political solution," the senior administration official said. "Until we understand how these arms promote a political solution, we do not see how provision of arms is a good idea."
Not much is known about al-Nusra's structure or leadership, but its emir, who goes by the nom de guerre of of Abu Muhammad al-Golani, said in an audio message at the beginning of the year that its goal was to make Syria an Islamic state. The group's propaganda frequently tells Syrians that they have been "abandoned" by the West.
The lesson of history is that revolutions tend to be won by the best organized and most ruthless, rather than the best intentioned and most reasonable. For Vladimir Lenin and the Bolsheviks, organization was as important as ideology. The Taliban swept across Afghanistan as the only coherent force in a country craving order and stability. Egypt's revolution ended up favoring the Muslim Brotherhood as the only political organization tried and tested by decades of repression. Similarly, al-Nusra stands out among Syrian rebel groups as the most effective and disciplined.
Al-Nusra has also learned from history, according to other Islamists among Syrian rebel ranks. Many of its more experienced fighters saw al Qaeda in Iraq alienate the Sunni tribes there with its vicious sectarian bloodletting. As Aaron Zelin of the Washington Institute for Near East Policy wrote in Foreign Policy last week, "Jabhat al-Nusra seems to have learned from the mistakes of al Qaeda in Iraq: It has not attacked civilians randomly, nor has it shown wanton disregard for human life by publicizing videos showing the beheading of its enemies."
Instead, Islamist leaders in Aleppo say, it is looking to broaden its appeal and spread its fundamentalist message.
Ordinary people in Aleppo complain that some FSA units mishandle or misappropriate bread distribution, and they have begun to look to Islamist groups for greater honesty in managing local affairs. One chant recently heard in Aleppo, revealingly, was this: "We don't want a thieving free army, we want an Islamic army."
The United States would rather see that role in the hands of the Local Coordination Committees, present in many Syrian cities since early in the uprising.
Such is the undercurrent of resentment toward the United States among Syrians pummeled by nearly two years of regime attacks that designating al-Nusra may have the unintended consequence of making it more popular. Designation may also make it more attractive to foreign jihadists. According to the SITE Intelligence Group, another jihadist group, the al-Sahaba Army, has already congratulated al-Nusra for its designation, describing it as a "great honor" and calling on other groups to rally to al-Nusra.
For now, the group has no formal affiliation with al Qaeda, and according to other Islamist fighters it is probably wary of being pigeonholed as part of the group, for fear of causing a backlash among a population that historically is among the less conservative in the Arab world.
One leading Islamist in Aleppo described al-Nusra as similar to jihadist groups in Iraq before they morphed into al Qaeda. So far, he said, al-Nusra has yet to receive al Qaeda's seal of approval.
The U.S. State Department thinks otherwise.