Then, on Wednesday, the Senate Foreign Relations committee voted by 10 members to seven to authorize action in Syria -- albeit under tighter rules of engagement than the White House had requested.
"None of us want the US mired down in another conflict, so the committee has significantly limited the president's original authorization, while still providing for an appropriate use of force in response to Assad's use of chemical weapons," said Republican Senator Bob Corker.
But for Obama to collect on the huge gamble he made in seeking congressional backing for attacks in Syria, he must win over lower ranking lawmakers who, unlike their leaders, are more concerned with their political skins than US standing in the world.
War weariness stalks America, and votes to authorize action in Syria, likely to begin next week, are tough ones -- especially for House and Senate lawmakers up for re-election in 2014.
Republicans face a strain of isolationist and libertarian sentiment roiling a party still working through the political trauma of the Iraq war.
Many establishment Republicans have already been knocked off their perches by "Tea Party" candidates who have challenged them from their right in nominating contests.
Anti-war liberal Democrats meanwhile are making unlikely common cause with conservatives like Senator Rand Paul, who oppose US military action.
Hawks like Republican senators Lindsey Graham and John McCain meanwhile demand a more robust effort than the "limited" strikes Obama has proposed and want to escalate US military aid to the Syrian opposition.
This splintering of party lines is complicating efforts to build a coalition behind action in Syria.
But one White House official predicted sufficient support would emerge between the extremes.
National Security Advisor Susan Rice told NBC News that she was "quite confident" that the administration would prevail.
Another official admitted privately that the White House "will lose some Democrats" meaning that significant numbers of Republicans will be needed not just in the House, but in the Democratic-run Senate to get the bill through.
Reliable vote counts are not yet available, as hearings on the authorization, featuring a passionate Secretary of State John Kerry take place on Capitol Hill.
But a top House Democrat, Chris Van Hollen, when asked by CNN Wednesday whether a war resolution could get through the chamber, replied: "I don't think anybody knows right now."
A Democratic House aide added: "There will have to be significant votes on both sides of the aisle."
Though Boehner and Cantor are in favor -- other top Republicans are wavering and could syphon away yes votes.
Republican House whip Kevin McCarthy said Wednesday he was "not there yet."
Another key figure, Paul Ryan, who like McCarthy may have leadership ambitions, is yet to tip his hand.
The White House has good reason for concern. Close Obama aides privately vent frustration that Boehner has been unable to deliver his riotous caucus on other big issues, including on proposed budget deals.
Democratic House minority leader Nancy Pelosi, a veteran vote counter, will be crucial.
She appeared to be speaking to liberal Democrats mulling a vote against their own president, when she made a humanitarian case for action on Tuesday.
Van Hollen meanwhile has been drafting a House resolution that would again constrain Obama's options, underlining anti-war feeling in the chamber.
Obama, often criticized for a failure to engage on key priorities on Capitol Hill, has been unusually active.
A senior official told AFP the president was calling lawmakers from abroad. On Wednesday, he also hardened his rhetoric.
"My credibility is not on the line, the international community's credibility is on the line and America and Congress's credibility is on the line," he said.
The comments appeared to be a bid to give Republicans, many of whom defy him on principle, a reason to vote 'yes' other than the fact the president's prestige is at stake.
He was also issuing a veiled threat of blame he will heap on lawmakers if the vote goes down.
Obama, Vice President Joe Biden, Rice and other key officials are also making an argument to lawmakers that if action is not taken against Syria, Iran will draw a dangerous lesson about American resolve, a senior administration official said.