WASHINGTON — Congress is holding its first public hearing about U.S. plans for military intervention in Syria as President Barack Obama seeks to convince skeptical Americans and their lawmakers about the need to respond to last month’s alleged sarin gas attack outside Damascus.

Secretary of State John Kerry, Defense Secretary Chuck Hagel and Joint Chiefs Chairman Gen. Martin Dempsey were to appear before the Senate Foreign Relations Committee today. A classified briefing open to all members of Congress was to take place as well.

The president’s request for congressional authorization for limited military strikes against Syrian President Bashar Assad’s regime is at the heart of all the discussions planned in Washington over the next several days as Obama sends his top national security advisers to the Capitol for a flurry of briefings. And with the outcome of any vote in doubt in a war-weary Congress, Obama was to meet today with leaders of the House and Senate armed services committees, the foreign relations committees and the intelligence committees.

Obama won conditional support Monday from two of his fiercest foreign policy critics, Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina.

A congressional vote against Obama’s request “would be catastrophic in its consequences” for U.S. credibility abroad, McCain told reporters outside the White House following an hour-long private meeting with the president.

But despite Obama’s effort to assuage the two senators’ concerns, neither appeared completely convinced afterward. They said they’d be more inclined to back Obama if the U.S. sought to destroy the Assad government’s launching capabilities and committed to providing more support to rebels seeking to oust Assad from power.

“There will never be a political settlement in Syria as long as Assad is winning,” Graham said.

McCain said today he is prepared to vote for the authorization that Obama seeks, but the Arizona Republican also said he wouldn’t back a resolution that fails to change the battlefield equation, where Assad still has the upper hand.

In an appearance on NBC’s “Today” show, McCain called it “an unfair fight” and said that if the authorization for U.S. military intervention doesn’t change the balance of power, it “will not have the desired effect.”

Sen. Robert Menendez, D-N.J., chairman of the Senate Foreign Relations Committee, said he believes the panel will back Obama if the administration explains “the full case” for the use of force as well as what it sees as the end result. “Not acting has huge consequences,” Menendez said on “CBS This Morning” today.

“It sends a message” not just to Syria, he said, but to Iran, North Korea and terrorist groups.

After a decade of war in Iraq and Afghanistan, polls show most Americans opposed to any new military action overseas.

That reluctance is being reflected by senators and representatives, some of whom say Obama still hasn’t presented evidence that Assad’s forces were responsible for the Aug. 21 attack that U.S. intelligence says killed 1,429 people, including more than 400 children. Others say the president hasn’t explained why intervening is in America’s interest.

Sen. Chris Coons, D-Del., said the administration needs to make its case on these points, if only to counter the misinformation and conspiracy theories circulating about Obama’s plans.

“Several people asked me if we were only interested in getting Syria’s oil,” Coons, a member of the Senate Foreign Relations Committee, said in a telephone interview with The Associated Press. “It’s important that Americans get the facts.”

Obama is to leave for a three-day trip to Europe tonight, visiting Stockholm, Sweden, and then attending an economic summit in St. Petersburg, Russia.