WASHINGTON — President Obama worked 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 after 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. In a conference call with administration officials, several Democrats pushed back against military action, questioning both the intelligence about a chemical attack last month outside Damascus and the value of an intervention to U.S. 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.

A French intelligence report on Monday alleged that the Syrian regime launched an attack on Aug. 21 that involved a “massive use of chemical agents,” and said it could carry out similar strikes in the future.

The government, on its website, published a nine-page intelligence synopsis about Syria’s chemical weapons program that found that at least 281 deaths could be attributed to the attack in rebel-held areas outside Damascus. The analysis based that count in part on dozens of videos culled by French intelligence services.

The extract said “the analysis of intelligence that we possess today leads us to estimate that on Aug. 21, 2013, the Syrian regime launched an attack on some areas of the Damascus suburbs held by opposition units, bringing together conventional means and the massive use of chemical agents.”

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, facing 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” on how they’ll vote on Obama’s authorization request, 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 Tuesday night for a three-day trip to Europe, visiting Stockholm and 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 that 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 U.S. 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 that 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.

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

After 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.

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 to 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.”