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The Obama administration, bolstered by evidence the Syrian government used lethal sarin gas on its own people, expressed confidence Sunday that Congress would back President Obama’s decision for a military strike on the Middle East country.

However, the president and his inner circle worked furiously over the weekend to win congressional support, appearing on Sunday shows, holding classified briefings and making calls to Capitol Hill leaders.

A senior administration officials told Fox News that the president, Vice President Joe Biden and Chief of Staff Denis McDonough made phone calls on Sunday to senators and House members urging them to vote in favor of the authorization of military force in Syria.

The official called the lobbying effort a "flood the zone" strategy, in an apparent acknowledgement of just how hard winning Capitol Hill approval will be.

The effort was preceded by Secretary of State John Kerry blanketing the Sunday shows and administration official proceeding with a round of weekend briefings, as Capitol Hill lawmakers said Obama may not have the votes right now.

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    “I would say if the vote were today, it would probably be a no vote,” New York Rep. Peter King, a member of the House Permanent Select Committee on Intelligence, told “Fox News Sunday.”

    The comment underscored the risk Obama took in deciding over the weekend to seek approval from Congress, a step King argues he didn't need to take.

    Kerry told “Fox News Sunday” that he couldn’t imagine Congress would “turn its back” on Israel or other U.S. allies in the region and on the Syrians slaughtered in the Aug. 21 chemical weapon attack, allegedly ordered by President Bashar al-Assad.

    “I can't contemplate that Congress would turn its back on all of that responsibility,” Kerry said. “I don’t believe Congress will do that.”

    Kerry’s comments and the White House blitz were the latest in a series of dramatic turn of events since the chemical weapon attack two weeks ago outside Damascus that killed more than 1,400 people including hundreds of women and children.

    The White House official confirms on Sunday the president called Arizona Republican Sen. John McCain over the weekend to invite him to the White House on Monday, the same day Obama will hold another conference call with House members.

    "In all calls and briefings, we will be making the same fundamental case: the failure to take action against Assad unravels the deterrent impact of the international norm against chemical weapons use, and it risks emboldening Assad and his key allies – Hezbollah and Iran – who will see that there are no consequences for such a flagrant violation of an international norm,” the official told Fox News.

    On Saturday, with Navy ships in the Mediterranean Sea ready to launch missiles, Obama made the surprising announcement Saturday, saying he had decided on a limited military response but would seek Congress’ approval.

    The announcement followed Kerry’s impassioned speech Friday for punishing Assad, whom he called a “thug” and a “criminal.”

    However, the largely Republican opposition to the strike had already taken shape and continued Sunday.

    Kentucky Republican Sen. Rand Paul, a member of the Senate Foreign Relations Committee, told NBC’s “Meet the Press” that the U.S. getting involved in the roughly 2-year-long Syrian civil war is a mistake and that the president has about a “50-50” chance of getting House approval.

    Democrats also expressed reservations.

    "I certainly enter this debate as a skeptic, but I'm going to allow the administration to make its case this week," Connecticut Democratic Sen. Christopher Murphy, a member of the Senate Foreign Relations Committee, told NBC.

    The administration’s classified briefings Saturday and Sunday included Republicans and Democrats in the GOP-controlled House and the Democratic-controlled Senate, where Obama will likely have the strongest support for the  biggest foreign policy vote since Congress authorized President George W. Bush to invade Iraq.

    More classified meetings are scheduled in the coming days on Capitol Hill, as party leaders consider whether to call members back from their August recess that ends Sept. 9.

    The Senate Foreign Relations Committee plans to meet Tuesday.

    Kerry, who appeared on all five major Sunday talk shows, said support from Capitol Hill and the public will give American the legitimacy a “full-throated” response.

    “Our country is much stronger when we act together,” said Kerry, knocking back speculation that Obama’s weekend announcement went against the advice of his national security team.

    “No decision is made until the president of the United States makes the decision,” he said.

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

    In an apparent attempt to win congressional support, he said the United States has received hair and blood samples from first responders indicating sarin was used in the 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 killings.

    Washington has struggled to rally allies to its cause, with only France firmly on board among major military powers. Britain cannot be counted on after Parliament rejected using force in a vote last week.

    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.

    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.

    Lawmakers told Fox News after a briefing Sunday that Congress will likely revise the resolution before voting on it. Among their concerns was that the words "targeted" and "limited" were not included, though the president has repeatedly said that would be the scope of such an attack.

    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.

    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.

    The Associated Press contributed to this report