WASHINGTON — The Obama administration began a full-press campaign on Sunday for Congressional approval of its plan to carry out a punitive strike against the Syrian government.

The lobbying blitz stretched from Capitol Hill, where the administration held its first classified briefing on Syria open to all lawmakers, to Cairo, where Secretary of State John Kerry reached Arab diplomats by phone in an attempt to rally international support for a firm response to the Aug. 21 chemical weapons attack in the suburbs of Damascus.

Mr. Kerry appeared on five morning talk shows, announcing new evidence — that the neurotoxin sarin had been used in the attack that killed more than 1,400 people — and expressing confidence that Congress would ultimately back the president’s plan for military action.

Behind closed doors on Capitol Hill, the administration presented classified intelligence to any senator or House member who wished to attend. About 80 did, but some from both parties emerged from the briefing convinced that the draft language authorizing military action would need to be tightened.

The rush of activity came a day after Mr. Obama’s surprise decision to seek the authorization of Congress for a strike on the Syrian government.

Ahead of an Arab League meeting in Cairo, Mr. Kerry sought to mobilize backing for American-led military action at a meeting the group held on Sunday night.

A statement that was issued by the league asserted that the Syrian government was “fully responsible” for the chemical weapons attack and asked the United Nations and the international community “to take the necessary measures against those who committed this crime.”

To the satisfaction of American officials, the statement did not explicitly mention the United Nations Security Council or assert that military action could be taken only with its approval. But it stopped short of a direct call for Western military action against Syria.

Before the meeting got under way, the Saudi foreign minister, Prince Saud al-Faisal, urged the international community to stop the Syrian government’s “aggression” against its people.

Saudi Arabia has been one of the principal supporters of the Syrian opposition, and Mr. Kerry consulted by phone on Sunday with Prince Bandar bin Sultan, Saudi Arabia’s intelligence chief and secretary general of its national security council.

The Obama administration’s calculation has been that a call for tough action by the Arab diplomats would enable the White House to argue to members of Congress that it had regional backing for military action and would make up, at least politically, for the British decision on Thursday not to join the American-led attack.

But Syria’s government on Sunday defiantly mocked Mr. Obama’s decision to turn to Congress, saying it was a sign of weakness. A state-run newspaper, Al Thawra, called the action “the start of the historic American retreat” and said Mr. Obama had put off an attack because of a “sense of implicit defeat and the disappearance of his allies.”

Syria’s deputy foreign minister, Faisal Mekdad, told reporters in Damascus, “It is clear there was a sense of hesitation and disappointment in what was said by President Barack Obama yesterday. And it is also clear there was a sense of confusion, as well.”

In some measure, part of the challenge that the Obama administration faces in trying to rally support at home for a punitive strike in Syria is the result of the deep ambivalence it has expressed about becoming involved in the conflict.

Part of the White House strategy for securing Congressional support now is to emphasize not only what Syria did, but also how a failure to act against Syria might embolden enemies of Israel like Iran and Hezbollah.

Mr. Kerry, in his television appearances, said that if Congress passed a measure authorizing the use of force, it would send a firm message to Iran that the United States would not tolerate the fielding of a nuclear device, and thus safeguard Israel’s security.

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“I do not believe the Congress of the United States will turn its back on this moment,” Mr. Kerry said on the NBC News program “Meet The Press.” “The challenge of Iran, the challenges of the region, the challenge of standing up for and standing beside our ally, Israel, helping to shore up Jordan — all of these things are very, very powerful interests and I believe Congress will pass it.”

One administration official, who, like others, declined to be identified discussing White House strategy, called the American Israel Political Affairs Committee “the 800-pound gorilla in the room,” and said its allies in Congress had to be saying, “If the White House is not capable of enforcing this red line” — against catastrophic use of chemical weapons — “we’re in trouble.”

Israeli officials have been concerned by Mr. Obama’s decision, but have been mostly restrained in their public comments. Mr. Kerry talked on Sunday with Benjamin Netanyahu, the Israeli prime minister.

Both the House and Senate are expected to have votes sometime after they return from recess on Sept. 9, although Senator Harry Reid, the Democratic leader, said the Senate Foreign Relations Committee would convene hearings on the Syrian issue Tuesday afternoon.

While Mr. Kerry said he was confident Congress would vote to approve the use of force, Representative Peter T. King, the New York Republican and a former chairman of the House Homeland Security Committee, said that if a vote in the House were held today, Mr. Obama would likely lose as a result of the “isolationist wing.”

Much of the debate in Washington concerned the terms of the resolution the White House has proposed for authorizing the use of force.

Representative Chris Van Hollen, a senior Democrat from Maryland, said that while the administration’s resolution limited the purpose of an attack to stopping the use of weapons of mass destruction, the measure left the military too much “running room” and did not set limits on the duration of the military operation.

Congressional advocates of strong action to help the Syrian opposition, in contrast, have complained that the attack that President Obama appears to be planning seemed to be too limited to have enough of an impact.

Senators John McCain of Arizona and Lindsey Graham of South Carolina, both Republicans, have warned that they would not support “isolated military strikes in Syria” that were not part of a broader strategy to shift the momentum on the battlefield. Mr. Obama is scheduled to meet with Mr. McCain on Monday at the White House.

As the White House consults with Congress, Mr. Kerry is planning a new round of diplomacy. He is planning to meet next weekend with European Union diplomats in Vilnius, Lithuania, and with Arab League diplomats in Rome.

After Mr. Obama’s change in direction, the reaction in Britain and France has largely been one of surprise and confusion. The French government, which had said on Friday that it would support a military strike, said it would wait for the American Congress to vote before taking any military action.

President François Hollande still intends to proceed with a military intervention of some kind in Syria, French officials said Sunday, but France will await the decision of Congress before taking action.

“We cannot leave this crime against humanity unpunished,” said Interior Minister Manuel Valls, speaking on French radio. But given logistical questions of “intervention capacity,” Mr. Valls said, France must “await the decision of the United States.”

“France cannot go forward alone,” he said. “There must be a coalition.”

A major question for military experts is what effect the delay in acting might have if force was eventually used by the United States.

Jack Keane, a former vice chief of staff of the Army and a retired four-star general, said in an interview that time would work to the advantage of President Bashar al-Assad as the Syrian forces would have more opportunities to move artillery, missiles and other equipment into civilian areas that they knew would not be struck.

Even Syrian command centers that could not be moved, he said, would be emptied of sensitive equipment and personnel.

But Mr. Obama said that he had been assured by Gen. Martin E. Dempsey, the chairman of the Joint Chiefs, that a delay would not affect the United States military’s ability to carry out a strike.