Barack Obama began personally calling wavering US lawmakers during his trip to Russia on Thursday as his pursuit of congressional authorisation for military action against Syria threatened to drag on well into next week.
The majority of Congress remains undecided, according to various unofficial "whip counts" collated in Washington, and the president has cancelled a trip to California planned for next week to "to work on the Syrian resolution before Congress", according to a White House official.
Obama is also expected to make a direct to appeal to the American people in a televised address from the oval office when he returns from the G20 summit in Russia.
Although the White House received a boost on Wednesday when an influential Senate committee narrowly voted in favour of military action, the introduction of clauses calling for regime change by hawkish Republicans has complicated efforts to secure support among more liberal Democrats.
"I am firmly undecided at this point," said senator Susan Collins, Republican of Maine, on Thursday. "This is a very difficult issue."
A running tally maintained by CNN estimated 24 senators were preparing to vote yes against 17 leaning no, with the remaining 59 undecided. In the House, the position appears reversed with 97 leaning against and only 28 so far declared in favour.
The perceived lack of domestic support for US intervention is proving
to be a major impediment to the administration's efforts to persuade
Congress.
Four recent polls have shown a majority of Americans oppose the idea of retaliatory airstrikes, with one poll last week by the Washington Post and ABC News showing 59% are against the White House strategy versus 36% in favour.
The administration was also put on the back foot Thursday by a New York Times report detailing alleged atrocities by Syrian rebel fighters.
State Department spokeswoman Jen Psaki condemned the apparent attack and said the US was seeking more information from rebel commanders.
She angrily rejected Russian claims, however, that secretary of state John Kerry had lied to Congress by downplaying the role of extremists in the rebel forces. Russian president Vladimir Putin claimed Kerry had denied any al-Qaida involvement, but the State Department said he was just saying its extent was exaggerated.
"Secretary Kerry is a decorated combat veteran who has had more than words aimed at him so he is not losing sleep over such as preposterous comment which was based on a misquote anyway," said Psaki.
She also insisted the administration remained confident of securing congressional backing for a response to alleged Syrian chemical weapons use. "Based on our conversation with members of Congress we are confident that they are not going to let this brutal act go unanswered," said Psaki.
Kerry will fly to Paris and London this weekend to help shore up international support for action against Syria amid various reports about the size of the coalition aligned with the US.
New UN ambassador Samantha Power, a noted advocate of humanitarian intervention, will also step up the administration's public lobbying on Friday with a speech to Center for American Progress in Washington expected to draw on her experiences of military action in the Balkans.
Psaki said at least 80 countries or organisations had acknowledged and condemned the use of chemical weapons in Syria – more than 50 of these publicly. A further 30 or more have said Assad was responsible, according to the State Department, and nine were in support of US military action: Australia, Albania, Kosovo, Canada, Denmark, France, Poland, Romania and Turkey. An unspecified number had offered military support of their own, but the US says it has the capacity to act alone if necessary.
