President Barack Obama is hoping his opponent in the 2008 presidential election will help sell the idea of US military intervention in Syria to the US nation.
Having announced over the weekend that he will seek congressional approval for military strikes against the Assad regime, the Obama administration is now trying to rally support among Americans and their congressman and senators - including the key figure of John McCain.
He has invited Mr McCain to the White House to address concerns of those who feel he is not doing enough to punish Bashar Assad's government for the chemical weapons attack in Damascus. On the other side of the coin, some Republican and Democratic lawmakers don't want to see military action at all.
Mr Obama's turnabout on Syria sets the stage for the biggest foreign policy vote in Congress since the Iraq war.
Secretary of State John Kerry said the US received new physical evidence in the form of blood and hair samples that shows sarin gas was used in the August 21 attack and the US must respond with its credibility on the line.
"We know that the regime ordered this attack," he said. "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."
Mr Kerry's assertion coincided with the beginning of a forceful administration appeal for congressional support.
At Congress, senior administration officials briefed politicians in private to explain why the US was compelled to act against Assad. Mr Obama, vice president Joe Biden and White House Chief of Staff Denis McDonough also made calls.
Mr McCain, one of the most hawkish members of Congress on foreign policy, said Mr Obama wanted him specifically to discuss Syria. "It can't just be, in my view, pinprick cruise missiles," he told CBS' "Face the Nation."
Mr Obama is trying to convince Americans and the world about the need for action. So far, he is finding few international partners willing to engage in a conflict that has claimed more than 100,000 lives and dragged in terrorist groups on both sides of the battlefield.