Secretary of State Hillary Rodham Clinton pronounced the Syrian National Council a failure last month. She said the United States and its partners would help the opposition unite “behind a shared, effective strategy that can resist the regime’s violence and begin to provide for a political transition that can demonstrate, more clearly than has been possible up until now, what the future holds for the Syrian people once the Assad regime is gone.”

Members of the Syrian National Council fought back like a fish on a hook, maneuvering to avoid what members feared would be marginalization. Members gathered in Doha this week to introduce changes — including doubling the group’s membership to more than 400, with about 33 percent of members from inside Syria, up from 15 percent. But some attempts to prove its diversity backfired; for example, not a single woman won in the elections for a 40-member secretariat.

The main criticism of the council, founded last fall, is that it has failed to attract the support needed to shift the balance of power away from Mr. Assad, instead spending months jockeying over internal positions. The council lacks significant support from Mr. Assad’s minority Alawite sect, as well as other minorities, tribal elders, religious figures and business groups.

“We will not have a vehicle for the future of Syria without those,” said Salman Shaikh, the director of the Brookings Doha Center, which helped lead the process of reshaping the opposition. “They don’t trust it.”

Syrian National Council members argue that they never got the financial or military support needed to attract a wider membership. But the group’s foreign backers calculated that with no end in sight to the fighting that has claimed nearly 40,000 people, by opposition estimates, it was time for a new approach. The longer-term goal is to convince Moscow of a credible alternative to Mr. Assad.

Participants here, meanwhile, made no secret of the fact that they want to get Washington more involved. The Obama administration, extracting itself from long wars in Iraq and Afghanistan, has been adamant that it would not do more than provide nonlethal aid to the Syrians. David Cameron, the British prime minister, said this week that he would work with the administration to make the opposition more effective.