WASHINGTON — Facing faltering support in foreign capitals and Congress for a strike against Syria, the Obama administration on Friday made an aggressive and coordinated push to justify a military intervention on the grounds that American credibility was at stake.

One day after the British Parliament voted against an attack on Syria, a stunning blow to White House plans for a broad coalition to punish President Bashar al-Assad of Syria for a mass killing in the suburbs of Damascus last week, President Obama and his top aides gave every indication that they were in final preparations for an attack that could pull the United States into a grinding civil war that has already claimed more than 100,000 lives.

Privately some American officials acknowledged mistakes over the past week in their buildup for a strike, not least misjudging the toxic politics of taking military action in the Middle East. It is unclear when Mr. Obama realized that the British vote would go against him, but it was not until Friday afternoon that the White House released what it said was evidence of chemical weapons use by the Assad forces — nearly 24 hours after Parliament had voted rather than beforehand, when it might have been used to build a coalition against Mr. Assad.

Deprived of the support of Britain, America’s most stalwart wartime ally, the Obama administration scrambled behind the scenes to build international support elsewhere for a strike that might begin as early as this weekend. Officials were still holding out hope that at least one Arab country might publicly join the military coalition.