Secretary of State John Kerry said on Sunday tests showed that sarin nerve gas was used in a deadly August 21 chemical attack near Damascus as he sought to build the case to convince skeptical lawmakers to authorize a military strike against the Syrian government.
He invoked the crimes of Adolf Hitler, Saddam Hussein and the potential threat to Israel from Syria and Iran in a round of television interviews a day after President Barack Obama delayed imminent military action in Syria to seek approval first from the U.S. Congress - a decision that puts any strike on hold for at least nine days.
It became apparent on Sunday that convincing Congress of atrocities committed by Syrian President Bashar Assad's forces was only one of the challenges confronting Obama as he seeks their approval.
Lawmakers raised a broad array of concerns, including the potential effectiveness of limited strikes, the possible unintended consequence of sparking a wider Middle East conflict, the wisdom of acting without broader international backing to share the burden and war-weariness of the American public.
Many in Congress have been able to avoid taking a position on the merits of a military strike, focusing instead on demands that Obama consult them and seek their approval.
Now that Obama has put lawmakers on the spot by demanding that they take a position, many appeared to be hedging.
While Kerry predicted Obama would win the endorsement he wants, a growing cacophony of congressional critics - ranging from liberal Democratic doves to Republican Tea Party conservatives - illustrated just how hard that will be.
"This is squarely now in the hands of Congress," Kerry told CNN, saying he had confidence "they will do what is right because they understand the stakes."
Kerry declined to say whether Obama would go ahead with military action if Congress rejects the president's request, as British parliament did last week to derail London's role in any Syria military operation. Echoing Obama's comments in the White House Rose Garden on Saturday, he insisted that the president had the right to act on his own if he chooses that course.
Obama is taking a gamble by putting the brakes on the military assault that he considers essential to maintain U.S. credibility after Assad crossed the "red line" set against the use of chemical weapons.
With lawmakers due to be briefed later on Sunday by Obama's national security team on the administration's rationale for military action, Kerry used the television appearances to provide further evidence backing its accusations against the Syrian government.
"I can share with you today that blood and hair samples that have come to us through an appropriate chain of custody, from east Damascus, from first responders, it has tested positive for signatures of sarin," Kerry told CNN's "State of the Union."
It was the first time the administration has pinpointed what kind of chemical was used in the attack on a rebel-held area, which U.S. intelligence agencies said killed more than 1,400 people, many of them children.
"So this case is building and this case will build," Kerry told NBC's "Face the Nation."
Obama's efforts are sure to be hampered by his dismal relations with congressional Republicans, who rarely miss an opportunity to oppose him. Another bitter face-off on government spending is looming this fall.
Lawmakers for the most part welcomed Obama's decision to consult them but looked in no hurry to come back to Washington early from their summer recess, which lasts until September 9.
Comments from leading Republicans and Democrats indicated how complex the debate will be - and raise doubts whether Obama will win their authorization.
Mike Rogers, Republican chairman of House of Representatives intelligence committee, told CNN: "I think there are some real challenges. I think that at the end of the day Congress will rise to the occasion. This is a national security issue."
Republican Senator Rand Paul of Kentucky, speaking on NBC, took a more skeptical view.
While saying that he was "proud" of Obama for coming to Congress for authority, Paul said: "It's at least 50-50 whether the House will vote down the involvement in the Syrian war."
"I think the Senate will rubber stamp what he wants," he said. "The House will be a much closer vote." The Senate is controlled by Obama's Democratic Party, the House is in the hands of the Republican Party.
Republican Senator John McCain said he was not sure Obama's request would pass but made clear his view that tougher military action was needed than the limited cruise missile strikes that the Obama administration is now preparing.
Republican congressman Peter King of New York said it was unclear if lawmakers would sign off on an attack on Syria but he warned Obama may have to overcome "the isolationist wing" of the Republican Party to prevail.
Seeking to lay the groundwork for what is expected to be a heated congressional debate, Kerry tipped his hand on one tactic the administration will use - linking the congressional vote to safeguarding U.S. ally Israel from the Syrian chemical weapons threat.
"I don't think they will want to vote, ultimately, to put Israel at risk," Kerry said.
Lawmakers of both major political parties recognize how important it is to be seen as defenders of Israel, especially at election time, when they compete to show voters who is a better friend of the Jewish state.
Additional reporting by Thomas Ferraro, Patricia Zengerle, David Brunnstrom; Writing by Matt Spetalnick; editing by Jackie Frank and Fred Barbash