“It’s not clear to me that the administration has a workable policy,” Senator Susan Collins, a Maine Republican on the Intelligence Committee, said last week.

Senior officials, including Vice President Joseph R. Biden Jr. and the C.I.A. director, John O. Brennan, have lobbied lawmakers in closed briefings and personal phone calls since late June. On Sunday, a senior administration official said that the Congressional concerns had been addressed and that “we look forward to pressing forward.” Some senior Congressional officials said Sunday that final details must still be worked out.

The Congressional impasse has exposed other shortcomings in the administration’s approach, lawmakers and independent Syria specialists said.

The slow start to the arming effort has led to skepticism — particularly as Mr. Assad’s troops retake strategically important towns from rebel forces — that the C.I.A.’s plan can achieve what Mr. Obama has said is America’s ultimate goal: forcing Mr. Assad to step down.

White House officials have made few public statements about the expanded military support to the rebels. It was not the president but Benjamin J. Rhodes, the deputy national security adviser for strategic communications, who announced the policy shift on June 13 in a conference call with reporters, saying that the approach “aimed at strengthening both the cohesion of the opposition, but also the effectiveness” of the rebels.

After the announcement, one senior Arab official said the United States would act like a “quarterback” — coordinating not only American arms shipments but also expanded deliveries of weapons from other allies, and probably providing opposition groups with intelligence reports on the movements of Syrian government forces. For nearly two years, a fractious coalition of Muslim nations, including Saudi Arabia, Qatar, Turkey and the United Arab Emirates, has supplied the rebels with weapons. The countries have been eager for the United States to take a direct role in arming them.

But even as American officials profess confidence that they can arm one segment of the opposition without empowering fighters from rebel groups like Jabhat al-Nusra, which the administration has designated as a terrorist organization, they face a daunting task in preventing arms from falling into the hands of extremists. They also acknowledge an immediate problem in trying to prop up one part of the opposition at the same time that an American ally, Qatar, is suspected of having supplied weapons to more hard-line Islamic groups in Syria, despite assurances to the contrary from Qatari officials.