Mr. Obama’s statements opening the door to the plan came as support for a resolution authorizing force on Capitol Hill seemed to be receding, with even some who had announced support for it now reversing course. Senator Harry Reid of Nevada, the majority leader, said Monday evening that he would not take the procedural steps to force an initial vote on authorization of force Wednesday, slowing Senate consideration of the resolution.

A senior Democratic aide said that the developments with the Russian proposal were a significant factor in the delay, which will allow members to consider the plan and also hear from the president, who is scheduled to meet with them at the Capitol on Tuesday in advance of his nationally televised speech to explain his rationale for military force.

Sergey V. Lavrov, the Russian foreign minister, made the proposal that could avert a strike earlier in the day, seizing on a seemingly offhand remark by Secretary of State John F. Kerry. Traveling in Britain, Mr. Kerry said that Bashar al-Assad, the Syrian president, could avoid strikes by agreeing to give up his chemical weapons.

“He could turn over every single bit of his chemical weapons to the international community in the next week — turn it over, all of it, without delay and allow the full and total accounting,” Mr. Kerry said.

Mr. Kerry’s remarks, especially the reference to the short window of time, underscored the urgency of the administration’s preparations for a strike, and it did not appear to signal a shift in policy. The State Department’s spokeswoman, Jen Psaki, later clarified in an e-mail to reporters that Mr. Kerry was simply “making a rhetorical argument about the impossibility and unlikelihood of Assad turning over chemical weapons he has denied using.”