By Elise Labott

Last Thursday night, John Kerry got a call from the White House. He had already spoken out Monday about what he said was “undeniable” evidence Syrian President Bashar al-Assad gassed his people. Now he was being asked to make a speech Friday afternoon laying out the case the United States had against the Assad regime and the importance of taking action to prevent him from doing it again.

In forceful and emotional remarks, drafted overnight by his staff and President Obama’s White House aides, Kerry laid out the U.S. intelligence against Assad and made the case for urgent U.S. action.

“The primary question is really no longer: What do we know,” Kerry said. “The question is: What are we – we collectively – what are we in the world going to do about it?”

His strong and, by most accounts, convincing indictment of Assad sounded so much like a case for imminent U.S. military action against Syria that the world was surprised when Obama hit the pause button the following day. In an address to the nation from the White House Rose Garden, the president explained his decision to seek congressional authorization for military strikes against Syria.

He had called Kerry Friday night to let him know about his decision, tasking him to help lead efforts to lobby Congress, allies and the American people to support U.S. action.

“Nobody, including Secretary Kerry, would say that the president’s decision didn’t come as a surprise,” one senior aide to Kerry said. “It was a, shall we say, unique decision. But he made it on his own and once he did the secretary and the rest of the national security team supported it.”

If he disagreed with the decision to walk back the march to war, Kerry didn’t show it. Over the next several days he sprang into action, talking on the phone nonstop with his former colleagues in the Senate and with his foreign counterparts to make the case for a limited U.S. intervention in Syria.

In five Sunday morning network television interviews and in close to eight hours of testimony on Capitol Hill, Kerry continue to be the administration’s spokesman on Syria, giving voice to the president’s decision to seek authorization from Congress and making the case that America could not stand by and allow the use of chemical weapons to go unpunished.

It is the moment, and burden, of a lifetime for the son of a diplomat after spending nearly three decades on the Senate Foreign Relations Committee coveting the job as secretary of state.

But it puts the nation’s diplomat further out on a limb as he tries to build support from skeptical allies who are concerned domestic politics will test Obama’s mettle.

“It put him in a difficult position,” acknowledged former Rep. Jane Harman, now president of the Woodrow Wilson Center for International Scholars. “The process was messy and not helpful. But he is loving every moment of this. Politics is a blood sport and John Kerry is used to this.”

The former presidential candidate-turned-senator came to prominence as a 27-year-old Vietnam War veteran who, fresh from the battlefield, gave emotional testimony against the war.

“How do you ask a man to be the last man to die for a mistake,” he asked the Senate Foreign Relations Committee in 1971. On Tuesday, as he addressed the same committee as the nation’s top diplomat, Kerry referred to his opposition to the Vietnam War as a reason for national debate on Syria.

Today, Kerry has become one of the administration’s most forceful advocates for action in Syria. Even as he has pushed for a diplomatic solution, aides say, Kerry has argued for stronger U.S. involvement in the Syrian crisis, even at one point raising the specter of airstrikes against regime targets.

In June, speaking about the U.S. efforts with Russia to broker talks between the Assad regime and the Syrian opposition for a political solution, Kerry offered muted criticism of the administration’s cautious policy, saying, "This is a very difficult process, which we came to late.”

“Throughout his career he has never held back from standing up for what he believes is right, whether it was around the Vietnam War or today for targeted action in Syria,” Jen Psaki, his spokesman at the State Department who worked with him on the 2004 presidential campaign, told CNN. “Making the case for the administration on what he feels personally is the best course, despite the political risk, the long hours of testimony and phone calls, is the heart of what he feels is his responsibility as secretary of state.”

And he still has his work cut out for him. This weekend Kerry makes a quick trip to Europe to meet with ministers from the Arab League and European Union, which has been divided about whether to join the United States in action against Syria. The memory of Iraq hangs heavily over both domestic and global public opinion, and the small size of the coalition as it stands reflects the complexity of the conflict itself.

“The world is wrestling with the same questions as we are,” said P.J. Crowley, a former State Department and National Security Council spokesman. “What will happen the day after the strike? Will Assad heed the lesson or will he retaliate? Everyone recognizes the problem but is divided with how to go about it.”

While Obama’s decision to seek authorization from Congress is billed as an effort to strengthen U.S. resolve, European and Arab diplomats say it is playing out as indecisive, which is causing countries to hedge their bets.

“Kerry is in a lose-lose position,” said Danielle Pletka, vice president of the American Enterprise Institute. “He is in the position of hopping around trying to situationally justify different decisions the president is making even as the president is changing them moment to moment. I hate to say that I feel sorry for him but I do.”

Aides say that Kerry understands the political risk, but relishes the opportunity to be the president’s front man on what is shaping up to be a defining legacy of his tenure, and he believes in the end the United States will come through.

“As long as we take action, and tough action, we think he will be in a good place in a few months,” one senior State Department official said. The official noted that one upside to the delay is that it will give Kerry more time for outreach to build international support.

Harman says that regardless of the outcome, Kerry’s star will rise.

“He has proven himself to be knowledgeable, colossally talented and loyal,” she said. “I think if someone gets hurt in the end, it is more likely to be the president himself. He is taking a big risk. But the outcome could be brilliant. We are putting on an international demonstration of what a mature democracy looks like. If this passes Congress, this play may turn out well for both the president and John Kerry.”