The US administration geared up for the biggest foreign policy vote since the Iraq war by arguing new evidence shows the Syrian government used sarin gas in a deadly August attack.
Members of Congress expressed sharply divergent opinions about whether to give President Barack Obama the go-ahead he requested to retaliate with military force against the Assad regime, and what turning down the commander in chief could mean for America's reputation.
Secretary of State John Kerry presented Mr Obama's case for military action in a series of interviews on TV news shows, outlining the latest information the administration has received about the August 21 attack in the Damascus suburbs that the US says killed 1,429 civilians, including more than 400 children.
He said samples collected by first responders in Damascus added to the growing body of proof that Syria's government had launched a chemical weapons attack.
"Samples of hair and blood have been tested and they have reported positive for signatures of sarin," Mr Kerry said. "Each day that goes by, this case is even stronger. We know that the regime ordered this attack. We know they prepared for it. We know where the rockets came from. We know where they landed. We know the damage that was done afterwards."
Sarin, which affects the nervous system and is toxic in liquid or gas form, can be delivered in missiles, bombs, rockets or artillery shells. The gas is outlawed under international rules of warfare. The reference to hair and blood samples were the first pieces of specific physiological evidence cited by any member of the administration, which previously spoke only about an unnamed nerve agent.
Mr Kerry's assertion coincided with the beginning of a forceful administration appeal for congressional support, now that Obama has declared he will await approval from the House of Representatives and Senate before ordering any cruise missile strikes or other action.
On Capitol Hill, senior administration officials briefed lawmakers in private to explain why the U.S. is compelled to act against President Bashar Assad's government. Further classified meetings were planned over the next three days.
Republican Sen John McCain, a leading Senate hawk and the candidate Mr Obama defeated for the presidency in 2008, said the president had invited him to the White House on Monday to discuss Syria.
Mr Obama must convince sceptical Americans and their representatives in Congress of the need for more U.S. military action in the Muslim world after a decade of war in Iraq and Afghanistan.