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WASHINGTON – U.S. President Barack Obama worked on Monday to persuade skeptical lawmakers to endorse a U.S. military intervention in civil war-wracked Syria, winning conditional support from two leading Senate foreign policy hawks even as he encountered resistance from members of his own party after two days of a determined push to sell the plan.

Sens. John McCain of Arizona and Lindsey Graham of South Carolina said Obama still needs to make a strong case for attacking the regime of President Bashar Assad, but they toned down past criticism that the president's plan was too weak to change the course of the fighting in Syria in favor of the opposition.

"We have to make it clear that a vote against this would be catastrophic in its consequences," now and in future international crises, McCain told reporters outside the White House following an hour-long private meeting that he and Graham had with Obama and White House national security adviser Susan Rice.

But the outcome of any vote remained in doubt amid continued skepticism in a war-weary Congress. Several Democrats in a conference call with administration officials pushed back against military action, questioning both the intelligence about a chemical attack last month outside Damascus and the value of an intervention to United States interests, according to aides on the call. Others demanded narrower authorization than that requested by the administration.

"The White House has put forward a proposed bill authorizing the use of force that, as drafted, is far too broad and open ended, and could be used to justify everything from a limited cruise missile strike to a no fly zone and the introduction of American ground troops," said Rep. Adam Schiff, D-Calif., a member of the House intelligence committee.

Obama has insisted that he will not send troops into Syria and that he was considering a military operation that was limited in duration and scope. The White House said Monday that Obama was open to working with Congress to make changes to the language of the resolution.

In a post on his website, Democratic Rep. Rick Nolan of Minnesota reflected a view shared by others: "I want you to know that I am vehemently opposed to a military strike that would clearly be an act of war against Syria, especially under such tragic yet confusing circumstances as to who is responsible for the use of chemical weapons."

After changing course and deciding to seek congressional approval for military action, Obama is confronted with one of his most difficult foreign policy tests and faces a Congress divided over an unavoidably tough vote-of-conscience on overseas conflict rather than the more customary partisan fights over domestic policy.

"My impression is that a lot of people are up for grabs," McCain said.

Following months of rejecting direct intervention in Syria, Obama and his aides now want to strike at the Assad regime in response to a reported chemical attack that the Obama administration says was carried out by Assad's military. The administration says more than 1,400 people were killed, including more than 400 children.

Obama was trying to find a middle ground that would attract a majority in the House and the Senate -- a difficult task complicated further because Obama is leaving for a three-day trip to Europe Tuesday night, visiting Stockholm, Sweden, and then attending an economic summit in St. Petersburg, Russia.

The visit is all the more significant because Russia has sided with the Syrian regime. Russia's foreign minister, Sergey Lavrov, said Monday the information the U.S. showed Moscow to prove the Syrian regime was behind the chemical attack was "absolutely unconvincing."

In a daring move, Russian President Vladimir Putin was considering sending a delegation of Russian lawmakers to the United States to discuss the situation in Syria with members of Congress, the Interfax news agency reported Monday.

The White House is engaging in what officials call a "flood-the-zone" persuasion strategy with Congress, arguing that failure to act against Assad would weaken any deterrence against the use of chemical weapons and could embolden not only Assad but also Iran and the Lebanese militant group Hezbollah -- an argument Obama reiterated in his meeting with McCain and Graham.

On Tuesday, Obama scheduled a meeting with leaders of the House and Senate armed services committees, the foreign relations committees and the intelligence committees.

On Tuesday afternoon, Secretary of State John Kerry, Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, are scheduled to testify publicly before the Senate Foreign Relations Committee. Earlier Tuesday, other members of the administration's national security and intelligence teams were to hold a classified, closed-door briefing for all members of Congress. A similar session was held Sunday and more will be held Thursday and Friday.

Kerry will also testify Wednesday before the House Foreign Affairs Committee. Kerry and Director of National Intelligence James Clapper will hold a classified briefing Wednesday with members of the Senate Foreign Relations Committee.

Members of the House Democratic caucus participated in an unclassified conference call Monday with Obama national security adviser Susan Rice, Kerry, Hagel, Clapper and Dempsey.

Following their white House meeting, McCain and Graham, who often speak in unison on foreign policy matters, said they were more inclined to back Obama's call for military action against Syria if it helps destroy the regime's missile launching capabilities and if the U.S. commits to provide more assistance to Syrian opposition forces.

"A degrading strike limited in scope could have a beneficial effect to the battlefield momentum," Graham said. "There will never be a political settlement in Syria as long as Assad is winning."

McCain, who was the Republican presidential nominee in 2008 and lost to Obama, said Obama clearly was asking for his help in rounding up votes. "I don't think he called us over because we're old campaign pals," he joked.

A senior state department official said Kerry called Syrian rebel commander Salim Idris on Monday discuss Obama's decision to seek congressional authority and to express confidence that U.S. military action would hold Assad accountable for the use of chemical weapons, deter his behavior and degrade the regime's ability to carry out such attacks. He also stressed the need for a "strong and unified moderate opposition."

As recently as Saturday, McCain and Graham issued a joint statement saying they could not support isolated military strikes that were not part of a broader strategy to change the momentum of the civil war and result in Assad's removal from power.

After Monday's meeting, McCain said: "Now we are talking about ways of approaching this issue in a way that could be effective. We've got to see more, but at least they are talking about some options that I think could work. "

Asked whether Obama would expand his targets in Syria, McCain alluded to the Navy's decision to place two aircraft carriers in the Arabian Sea. The USS Truman arrived in the region to take the place of the USS Nimitz, which was supposed to head home. But the Navy ordered the Nimitz to stay for now.

"I don't think it's an accident that the aircraft carriers are in the region." McCain said.

U.S. officials, however, have described the decision as prudent planning and have said it doesn't suggest the Nimitz would play a role in any possible strikes in Syria.

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Associated Press writers Bradley Klapper, Donna Cassata and Matthew Lee contributed to this report