A protesters holds up a poster of Syrian President Bashar al-Assad near placards reading "U.S. killer, get out the middle east" during a demonstration in Hatay, Turkey, Sunday, Sept. 1, 2013. The United States is considering launching a punitive strike against the Assad regime, blamed by the U.S. and the Syrian opposition for an Aug. 21 alleged chemical weapons attack in a rebel-held suburb of the Syrian capital of Damascus. (Gregorio Borgia/AP)

As the Obama administration launched what it described as a “flood the zone” campaign to persuade Congress to authorize military action against Syria, officials said Monday that they are willing to rewrite the proposed resolution to clarify that any operation would be limited in scope and duration and would not include the use of ground troops.

Their signal of flexibility came amid indications that Obama is picking up tentative, conditional support in what promises to be a difficult battle for approval of military strikes.

After meeting with the president Monday at the White House, Sen. John McCain (R-Ariz.) — who has been among the leading Syria hawks — warned that if Congress rejects the resolution, “the consequences would be catastrophic. The credibility of this country with friends and adversaries alike would be shredded, and there would be not only implications for this president but for future presidencies as well.”

As McCain’s comments suggest, lawmakers are approaching their decision with an awareness that its ramifications go well beyond the thorny question at hand, which is how to punish the government of Syrian President Bashar al-Assad for his alleged use of chemical weapons against his own people. A vote is expected shortly after Congress returns to Washington next week.

Obama’s surprising decision to seek congressional approval, announced Saturday, marks a turning point from recent history, in which presidents have given lawmakers less and less of a role in deciding when and how to take the country to war. Obama was elected in part as a rejection of the George W. Bush administration’s push to invade Iraq a decade ago.

As of Sept. 4, lawmakers appear to be tentatively dividing into four camps over military action in Syria.

Yet the negotiations and the outcome on Syria could set a precedent that limits the latitude of future presidents — or makes them wary of consulting Congress at all.

Obama has not said what he will do if Congress rejects the strike resolution. The White House maintains that it does not have to seek congressional authorization for the brief military campaign it envisions.

Preparations for that operation continued Monday. The aircraft carrier USS Nimitz and other ships in its strike group moved into the Red Sea to help support a strike, if needed, defense officials said.

Adding to the challenge of winning congressional approval is the fact that Obama is undertaking it at a time of hyper-partisanship that has produced gridlock on a host of domestic issues. White House officials say they hope that this vote can be elevated from the political morass and made on the merits — a view that has been echoed by influential Republicans, including House Intelligence Committee Chairman Mike Rogers (Mich.).

The administration’s openness to negotiating new language for its proposed resolution is an attempt to address concerns, voiced on the left and the right, that the draft is too vague. Doing so would also put into writing some of the reassurances that Obama has offered in his statements.

“To secure a yes from Congress, the White House will need to make a compelling intelligence presentation, have persuasive answers to questions about what it will do if the Assad government persists in using chemical weapons despite a U.S. military strike, and write authorizing language that is neither too expansive for skeptics nor too limited for hawks,” said James Lindsay, director of studies at the Council on Foreign Relations.

The current version of the resolution, sent to Congress on Saturday, would give the president authorization to use force against Syria “as he determines to be necessary and appropriate.”

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“It misses some language which says, “Here’s why we’re doing this, and here’s what success would look like,’ ” said Rep. Mike Pompeo (R-Kan.), a member of the House Intelligence Committee who said he is inclined to support Obama. “It needs some real benchmarks, real measurements that say, ‘Here’s what we’re trying to do.’ ”

Memories of the 2003 Iraq invasion — premised on faulty intelligence and uncertain goals — have added to Congress’s skepticism and its demands for clarity, added Rep. Gerald E. Connolly (D-Va.), who participated in a conference call Monday between top administration officials and 127 members of the House Democratic Caucus.

“Iraq’s shadow is so long, so pervasive, that people can’t look past it to see that this is not Iraq,” Connolly said. “That is particularly acute on my side of the aisle.”

Meanwhile, McCain and Sen. Lindsey O. Graham (R-S.C.) indicated that Obama has given them new assurances addressing their complaints that the Syrian battlefield is tilted against the forces opposing Assad’s government. They strongly suggested that the White House may further expand its help for the rebels, who now receive very limited U.S. military supplies and advice, and that the administration will be articulating a more comprehensive strategy this week.

“There seems to be emerging from this administration a pretty solid plan to upgrade the opposition, to get other regional players involved,” Graham said after their session with Obama on Monday.

McCain added: “Before this meeting, we had not had this indication. Now the question is whether that will be put into a concrete strategy that we can sell to our colleagues and that we agree with.”

The administration’s lobbying campaign is set to intensify on Tuesday. Obama is scheduled to meet with the leadership of the Senate’s Armed Services, Foreign Relations and Intelligence committees and their counterparts in the House.

Though Congress is not back in session from its August recess, Secretary of State John F. Kerry, Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, will make the case for U.S. military strikes in Syria in testimony before the Senate Foreign Relations Committee.

Hagel was a member of that panel and Kerry was its chairman when they were in the Senate; Obama and Vice President Biden also came to their jobs from the Senate and served on the committee. That experience helped shape the perspective all four bring to the question of the balance of power between the legislative and executive branches when it comes to foreign policy and military action.

Kerry plans to argue that “the failure to take action against Assad unravels the deterrent impact of the international norm against chemical weapons use, endangers our friends and our partners along Syria’s border” and risks emboldening Assad and his backers Iran and the Hezbollah militant network, said a senior State Department official who spoke on the condition of anonymity to outline Kerry’s main arguments.

Kerry and perhaps others also will appear before the House Foreign Affairs Committee on Wednesday.

Administration officials will hold classified briefings open to all lawmakers each day this week, beginning Tuesday morning, a senior administration official said Monday. The official requested anonymity to describe the White House strategy.

The Senate Foreign Relations Committee will also hold a closed, top-secret hearing on Wednesday, and several other panels in the House and Senate plan similar sessions over the next week.

Monday’s telephone briefing for Democrats reviewed the unclassified information the White House claims leads to an undeniable conclusion that Assad’s forces targeted civilians with chemical weapons in an Aug. 21 attack on the outskirts of Damascus. The Obama administration says that more than 1,400 were killed.

At least one lawmaker accused the administration of forgetting the lessons of Iraq and Vietnam, congressional aides said. Kerry led the administration’s counterargument that the United States should apply force that is “proportional” to the threat at hand, participants said.

Slightly over a year ago, Obama had set chemical weapons use in Syria as a “red line” for the United States.

Syria possesses one of the world’s largest inventories of chemical weapons, including sarin and other nerve agents banned by an international treaty that Syria has refused to sign. The Obama administration said Assad remains in control of the weapons.

The 21 / 2-year conflict in Syria has killed more than 100,000 and made refugees of close to 2 million Syrians. Close U.S. allies Jordan and Turkey are bearing much of the brunt of the humanitarian crisis.

Despite the humanitarian toll, Obama had not seen the Syrian crisis as a national security risk to the United States that was worth the risks and downsides of military action. That began to change in June, when Obama approved limited provision of U.S. weapons to the Syrian rebels in response to an initial, small-scale use of chemical weapons.

Not all the lobbying efforts involve Congress. Kerry spoke Monday to the head of Syria’s rebel military arm, Gen. Salim Idriss, the State Department said. Rebels supported the quick strike Obama had planned, and some were angered by what they saw as a bait-and-switch when he opted to hold off.

“The secretary expressed that the United States is confident that we can hold the Assad regime accountable for its use of chemical weapons, deter this kind of behavior and degrade the regime’s capacity to carry it out,” a State Department official said. “The secretary reaffirmed that we will continue to support the Syrian people.”

Kerry also spoke Sunday with Israeli Prime Minister Benjamin Netanyahu and several Arab diplomats, as well as France’s foreign minister. The United States is trying to beef up Arab support for a strike in the absence of a U.N. mandate.

Ed O’Keefe contributed to this report.

by Anne Gearan, Craig Whitlock

and Ed O’Keefe

Secretary of State John F. Kerry and Defense Secretary Chuck Hagel will make the case for U.S. military strikes in Syria in testimony Tuesday before the Senate Foreign Relations Committee, Senate aides said Monday.

The televised appearance will be one of the most public aspects of an intense lobbying effort underway by the Obama administration, which includes in-person briefings for lawmakers, phone calls and meetings at the White House.

The Senate Foreign Relations Committee will hold a closed, top-secret hearing on Wednesday, and several other panels in the House and Senate plan similar sessions over the next week.

A vote on the proposed punitive attack is expected sometime after Congress returns to formal session Sept. 9. Approval by a skeptical Congress is far from assured, with Democratic votes likely to determine the outcome. President Obama, elected in part as a rejection of the George W. Bush administration’s push for war in Iraq a decade ago, announced Saturday that he would put his case for a limited attack on Syria up for a vote.

On Monday, Kerry and Hagel were scheduled to join an 11:30 a.m. conference call with House Democrats to discuss the situation in Syria, according to senior Democratic aides. National Security Adviser Susan E. Rice and Director of National Intelligence James R. Clapper Jr. also were participating.

The administration asserted Sunday for the first time that the Syrian government used the nerve gas sarin to kill more than 1,400 people. The alleged Aug. 21 attack is thought to be the world’s gravest chemical weapons attack in 25 years.

Kerry said new laboratory tests showed traces of sarin, an extremely toxic nerve agent, in blood and hair samples collected from emergency workers who responded to the scene of the alleged attack in the Damascus suburbs.

Kerry and Hagel’s Tuesday appearance before the Foreign Relations Committee will be a return for both of them to a panel they served on as senators. Kerry was chairman of the committee before leaving the Senate to run the State Department.

The committee plans to meet at 2:30 p.m. Tuesday in the first of what is expected to be several Senate and House hearings on Syria and the request by Obama to authorize military force there.

The Reuters news agency reported Monday that the nuclear-powered aircraft carrier USS Nimitz and other ships in its strike group were heading toward the Red Sea to help support a strike, if it is approved.

In an interview blitz on five Sunday television news shows, Kerry said fresh forensic evidence strengthened an already compelling case for taking military action against the government of Syrian President Bashar al-Assad, and he predicted that Congress would vote to give Obama that authority.

“I can’t contemplate that the Congress would turn its back on all of that responsibility and the fact that we would have, in fact, granted impunity to a ruthless dictator to continue to gas his people,” Kerry said on “Fox News Sunday.” “Those are the stakes.”

But many lawmakers, including some who had previously pushed for a harder line against Assad, said the White House would have a tough time drumming up support for intervention in Syria’s seemingly intractable civil war.

“My constituents are war-weary. They don’t want to see us get involved in this,” Sen. Saxby Chambliss (R-Ga.) said on CBS’s “Face the Nation.” “The president has an obligation to make his case to the Congress, but he also has an obligation to make this case to the American people. . . . It’s going to be a very, very tough debate.”

It is a debate most in Congress did not expect. Administration officials had given every sign that Obama would order an attack on Syria over the Labor Day weekend. The Navy had moved destroyers armed with cruise missiles close to the Syrian coast, and Pentagon officials said they were ready to strike.

On Saturday, however, Obama stunned much of the world by announcing that he had decided to hold off for the moment and that he would seek formal backing from Congress. On Sunday, he dispatched a stream of national security officials to Capitol Hill for classified briefings with lawmakers who interrupted their summer recess to rush back to Washington.

Senate Judiciary Committee Chairman Patrick J. Leahy (D-Vt.) told reporters that Obama’s proposed resolution seeking authorization to use military force would not pass Congress as written because it was too broad. “It will be amended,” Leahy said.

Rep. Bill Pascrell Jr. (D-N.J.) said he sensed that colleagues in both parties appreciated the seriousness of the decision, but that many were genuinely undecided. “I’m glad I read the documents; it was worth the trip,” he said. “I haven’t really made up my mind. I’m not trying to be a wise guy — I just haven’t.”

Obama warned Assad a year ago not to use chemical weapons against his own people, saying such a move would cross a “red line” that could force the United States to intervene militarily. On Aug. 21, a torrent of grisly videos surfaced on the Internet, depicting mass casualties from a nighttime attack outside Damascus that rebel groups said involved poison gas.

In an unclassified intelligence dossier made public Friday, U.S. officials said they believe that the Syrian government used a “nerve agent” in the attack. But the intelligence report did not specify the kind of nerve gas and appeared to lack forensic evidence that would support a definitive conclusion.

U.N. report awaited

On Sunday, Kerry said U.S. officials had received fresh lab results showing traces of sarin in hair and blood samples collected from the scene. He did not give further details or elaborate on the source of the material, other than to say it had not come from a team of United Nations inspectors who left Syria on Saturday.

The U.N. team of chemical weapons experts had spent four days near the site of the alleged attack and brought an extensive collection of forensic samples and other evidence to the Hague for testing in European laboratories.

The results of those tests, depending on how quickly the tests are conducted, could play an influential role as Congress debates whether to endorse military action against Syria. If the U.N. tests confirm the use of sarin gas, they will lend independent credibility to the Obama administration’s arguments. If the U.N. tests are inconclusive or offer conflicting results, they might badly undercut the White House.

U.N. officials had said previously that it could take as long as two weeks for the experts to present their findings. On Saturday, a U.N. spokesman said, “Whatever can be done to speed up the process is being done, but we are not giving a timeline.”

Syria is thought to have multiple nerve agents and other poison gases in its chemical stockpile. Assad’s government has denied carrying out gas attacks, blaming rebel forces instead.

Long-standing global treaties have banned the use of chemical weapons. The last major chemical weapons attack occurred in 1988, in Halabja, Iraq, a Kurdish city that was gassed on the orders of ruler Saddam Hussein.

Both parties divided

Kerry said the purpose of a limited military strike would be to deter Syria from using chemical munitions, as well as to send a message to Iran and other countries in the region that Obama would back up his warnings about the development or use of weapons of mass destruction. Removing Assad from power or affecting the tide of Syria’s civil war was not the goal, administration officials said.

Kerry also played down the timing of any military action, saying it would not make any difference if it occurred a week or two later than originally planned.

Syrian opposition groups have reported that Assad’s forces have been moving troops and military equipment into civilian areas, hoping to shield their resources from a U.S. attack.

Current and former U.S. military officials said, however, that any missile strikes would probably be aimed at Syrian army installations, bridges and other fixed targets, rather than troops or mobile weapons systems. They said the Pentagon would almost certainly avoid attacks on chemical weapons depots because of the risk that poison gases would leak and disperse into the air.

“Most of the targets associated with this limited strike are fixed. They’re buildings, they’re facilities, they’re areas,” James E. Cartwright, a retired Marine general and former vice chairman of the Joint Chiefs of Staff, told ABC’s “This Week.” “They’re not going to move.”

Loveday Morris in Beirut and Paul Kane and Sean Sullivan in Washington contributed to this report.