The Obama administration on Sunday confidently predicted congressional backing for limited military action in Syria on Sunday and disclosed fresh evidence the Assad government used sarin gas in a deadly attack.

Senior U.S. officials sought to lay out their case to divided lawmakers in a classified briefing as the countdown began to the biggest foreign policy vote since Congress authorized President George W. Bush to invade Iraq.

In a series of interviews on the Sunday news shows, Secretary of State John Kerry said the case for intervention in Syria’s 2½-year civil war was strengthening each day and that he expected American lawmakers to recognize the need for action when the “credibility of the United States is on the line.”

He said President Barack Obama has the authority to launch retaliatory strikes with or without Congress’ approval, but Kerry stopped short of saying the president would do so if the House or Senate withholds support.

“The stakes are just really too high here,” Kerry said. “We are not going to lose this vote.”

Seeking to sharpen the argument for war, Kerry said the United States has received hair and blood samples from first responders indicating that Syria’s government forces used sarin in its Aug. 21 attack in the Damascus suburbs.

It was the first piece of specific physiological evidence cited by the administration, which previously cited only an unnamed nerve agent in the killing of 1,429 civilians, including more than 400 children. The U.S. says such chemical weapons use compels an international response.

Washington has struggled to rally allies to its cause, however, with only France firmly on board among major military powers. Stalwart ally Britain cannot be counted on after Parliament rejected using force in a vote last week. That could be a harbinger of the difficult task ahead for the Obama administration as it seeks Congress’ approval for cruise missile strikes and other limited measures once lawmakers return from summer break, which is scheduled to end Sept. 9.

A little more than a year ago, Obama declared that Assad’s use of chemical weapons would be the “red line” in a conflict that he has steadfastly avoided. But Obama deferred any immediate action Saturday by announcing that he first would seek congressional authorization.

Senior administration officials were briefing members of Congress in private later Sunday. Further classified meetings would be held over the next three days, Kerry said. The Senate Foreign Relations Committee plans a Tuesday hearing.

Navy ships are on standby in the Mediterranean Sea ready to launch missiles.

Obama is likely to find strong support in the Democrat-controlled Senate but a tougher fight in the GOP-dominated House.

Despite the intense gridlock on Capitol Hill over debt reduction, health care, immigration and other issues, at least some senior lawmakers said the president should be able to cobble together a strong enough coalition of moderate and hawkish members of both parties to defeat anti-war Democrats and tea party-backed Republicans opposed to intervention.

“We cannot make this about the president versus the Congress. This isn’t about Republicans versus Democrats. This has a very important worldwide reach,” said Rep. Mike Rogers, R-Mich., the chairman of the House Intelligence Committee. “At the end of the day, Congress will rise to the occasion,” he said, but added that “it’s going to take that healthy debate to get there.”

GOP Rep. Peter King of New York, who criticized Obama for not proceeding immediately against Assad, predicted a tougher challenge. “I think it is going to be difficult,” said King, citing the rising “isolationist” tendency among members of his party.

A leader of that faction, Sen. Rand Paul of Kentucky, said he expected the Senate to “rubber-stamp” what Obama wants. “But I think the House will be a much closer vote,” he said, saying the balance was “at least 50-50 whether the House will vote down involvement in the Syrian war.”

Late Saturday, the White House sent Congress a draft resolution authorizing force against Syria to “deter, disrupt, prevent and degrade” the Assad regime’s ability to use chemical weapons. It doesn’t lay out a timeline for action or detail Obama’s strategy.

Kerry reiterated Obama’s oft-repeated promise not to send any American troops into Syrian territory, a reflection of the president’s own aversion to getting too deeply involved in a conflict that has killed more than 100,000 people and drawn in terrorist organizations on both sides of the battlefield.

Polls also show significant opposition among Americans to involvement after a decade of war in the Muslim world, and several officials have cited the faulty intelligence about weapons of mass destruction that led up to the Iraq war as justification of the need for lengthy debate before U.S. military action.

Kerry, who voted to authorize Bush’s 2003 Iraq invasion but then opposed the war in his unsuccessful presidential bid a year later, rejected any comparisons to those conflicts.

“This is not Iraq. This is not Afghanistan. There is nothing similar in what the president is contemplating,” Kerry said. “There are others who are willing to fight, others who are engaged. And the issue here is not whether we will go and do it with them, it’s whether we will support them adequately in their efforts to do it.”

Asked repeatedly what Obama would do if Congress didn’t give its consent, Kerry said, “The president has taken his decision.”

“He is not trying to create an imperial presidency,” Kerry said. “I believe that in the end, Congress will do what is right.”

Administration officials said that until late Friday Obama appeared set on ordering a strike without first seeking Congress’ approval. After a walk around the White House grounds with Chief of Staff Denis McDonough, the president told his aide he had changed his mind.

Complicating efforts in Congress is the demand for even stronger action from Sens. John McCain of Arizona and Lindsey Graham of South Carolina, the leading Republican hawks in the Senate.

McCain said he and Graham and others “will be wanting a strategy, a plan, rather than just we’re going to launch some cruise missiles and that’s it.” McCain said he was invited to the White House by Obama to discuss Syria and planned to go there Monday.

Kerry appeared on NBC’s “Meet the Press,” CNN’s “State of the Union,” CBS’ “Face the Nation,” ”Fox News Sunday” and ABC’s “This Week.” Paul was on NBC, Rogers was on CNN, King was on Fox and McCain on CBS.

(AP)