In recent weeks, thousands of Syrians have recorded personal appeals to members of Congress and the American public urging them to oppose an airstrike, though it is not clear whether those efforts are coordinated with their government.

For the rebels, who could often use a tip or two in the area of public relations, all of this is unqualified bad news. “It is disappointing,” said Najib Ghadbian, the main Syrian opposition group’s special representative to the United States. “If the regime wants to play with this, it could take months or years. This is why we need accountability.”

A rebel brigade commander named Moaz al-Yousef, reached by telephone, spoke bitterly of Mr. Obama’s interest in the Russian proposal — and the delay of the Congressional votes — as a betrayal.

“We had hopes, it was a dream, and now it’s gone and we feel disappointed,” he said. “We should completely cut off our relationship with him — Obama has completely lost his credibility.”

The rebels’ foreign backers were almost equally derisive. Recep Tayyip Erdogan, the Turkish prime minister, dismissed the Russian proposal in a speech in Istanbul on Thursday, saying that Mr. Assad was merely buying time for “new massacres.”

In his interview with Russian television, Mr. Assad hinted at another possible stumbling block in the prospective chemical weapons agreement by saying Israel should ratify it first. Israel has signed the accord but not ratified it, and is extremely unlikely to do so in light of the difficulty of verifying Syrian compliance in the midst of a civil war.

For Mr. Assad, the Russian proposal comes as a welcome reprieve. Even before the chemical weapons attack on Aug. 21, his military was effectively locked in a stalemate with the opposition, despite the intervention of militia fighters from Hezbollah, the Lebanese Shiite movement, in recent months. Although Mr. Assad won a few important victories, he has still not pushed the rebels from the Damascus suburbs. That, many analysts say, was the goal of the chemical weapons attack, in a rebel-held part of the eastern suburb of Ghouta.