WASHINGTON -- President Barack Obama is inviting former foe Sen. John McCain to the White House, hoping one of Congress' most intractable foreign policy hawks will help sell the idea of a U.S. military intervention in Syria to a nation deeply scarred by more than a decade of war.

Having announced over the weekend that he'll 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.

Monday's meeting with McCain is meant to address concerns of those who feel Obama isn't doing enough to punish Syrian President Bashar Assad's government for an attack in the Damascus suburbs last month that the U.S. says included sarin gas and killed at least 1,429 civilians, more than 400 of whom were children. On the other side of the spectrum, some Republican and Democratic lawmakers don't want to see military action at all.

Obama's turnabout on Syria sets the stage for the biggest foreign policy vote in Congress since the Iraq war.

On Sunday, Secretary of State John Kerry said the U.S. received new physical evidence in the form of blood and hair samples that shows sarin gas was used in the Aug. 21 attack. Kerry said the U.S. 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."

Kerry's assertion coincided with the beginning of a forceful administration appeal for congressional support.

On Capitol Hill, senior administration officials briefed lawmakers in private to explain why the U.S. was compelled to act against Assad. Obama, Vice President Joe Biden and White House Chief of Staff Denis McDonough also made calls to individual lawmakers.

Further classified meetings were planned from Monday to Wednesday. The Senate Foreign Relations Committee plans a meeting Tuesday, according to its chairman, Sen. Bob Menendez, D-N.J. The Senate Armed Service Committee will gather a day later, said Oklahoma Sen. Jim Inhofe, the top Republican on the panel.

McCain, the candidate Obama defeated for the presidency in 2008, said Obama asked him to come to the White House specifically to discuss Syria.

"It can't just be, in my view, pinprick cruise missiles," the Arizona Republican told CBS' "Face the Nation."

In an interview with an Israeli television network, he said Obama has "encouraged our enemies" by effectively punting his decision to Congress. He and fellow Republican Sen. Lindsey Graham of South Carolina have threatened to vote against Obama's authorization if the military plan doesn't seek to shift the momentum of the 2 1/2 year civil war toward the rebels trying to oust Assad from power.

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 in the past 2 1/2 years and dragged in terrorist groups on both sides of the battlefield.

Only France is firmly on board among the major military powers. Britain's Parliament rejected the use of force in a vote last week.

With Navy ships on standby in the eastern Mediterranean ready to launch missiles, Congress on Sunday began a series of meetings that are expected to continue over the next several days in preparation for a vote once lawmakers return from summer break, which is scheduled to end Sept. 9.

Senior administration officials gave a two-hour classified briefing to dozens of members of Congress in the Capitol on Sunday.

Lawmakers expressed a range of opinions coming out of the meeting, from outright opposition to strident support for Obama's request for the authorization to use force.

Among Democrats, Rep. Sander Levin of Michigan said he'd approve Obama's request and predicted it would pass. Rep. Elijah Cummings of Maryland said he was concerned the authorization might be "too broad." Rep. Bennie G. Thompson, the senior Democrat on the House Homeland Security Committee, said the administration still has "work to do with respect to shoring up the facts of what happened."

Republican Rep. Cathy McMorris Rodgers of Washington said she was concerned about what Congress was being asked to approve. Sen. Jeff Sessions, R-Ala., said the war resolution needed tightening.

"I don't think Congress is going to accept it as it is," Sessions said.

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