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 of Staff Chairman Gen. Martin Dempsey were to appear before the Senate Foreign Relations Committee on Tuesday. 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 core of 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 Tuesday with leaders of the House and Senate armed services committees, the foreign relations committees and the intelligence committees.

The president announced over the weekend that he would seek approval from Congress for military strikes against the Assad regime to respond to an attack in the Damascus suburbs last month that the U.S. says involved deadly sarin gas.

That decision sets the stage for the biggest foreign policy vote in Congress since the Iraq war. A vote could come once lawmakers return from summer break, which is scheduled to end 9 September.

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 would be more inclined to back Obama if the US sought to destroy the Assad government's launching capabilities and committed to providing more support to rebels seeking to oust Assad.

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

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 bulletproof 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.

After a Labor Day holiday weekend spent listening to concerned constituents, Democratic Sen. Chris Coons said the administration needed to make its case, 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."

Petroleum is hardly the most pertinent question. Even before Syria's hostilities began, its oil industry contributed less than half a percent of the world's total output. And Obama has expressly ruled out sending American troops into Syria or proposing deeper involvement in the Arab country's violent civil war.

But such queries are a poignant reminder of the task awaiting the administration as it argues that the United States must exert global leadership in retaliating for what apparently was the deadliest use of chemical weapons anywhere over the past 25 years.

Obama has insisted 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 in the language of the resolution, which Congress was expected to begin considering next week.

In a conference call Monday with House Democrats, several members of Obama's own party challenged the administration's assertions. Some questioned the intelligence about the 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 for the use of force than that requested by the Obama administration, saying the proposed bill was too broad and open-ended.

In a post on his website, Rep. Rick Nolan reflected a view shared by at least some of his colleagues: "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."

Their skepticism is shared by many conservative tea party Republicans and others, whose views range from ideological opposition to any U.S. military action overseas to narrower fears about authorizing the use of force without clear constraints on timing, costs and scope of the intervention.

The most frequent recurring questions: How convinced is American intelligence about the Assad regime's culpability for the chemical attack, a decade after woefully misrepresenting the case that Iraqi President Saddam Hussein possessed weapons of mass destruction? And how does a military response advance U.S. national security interests?

Pressuring the administration in the opposite direction are hawks and proponents of humanitarian intervention among both Democrats and Republicans who feel what Obama is proposing is far too little.

Obama's task is complicated further because he is leaving for a three-day trip to Europe on 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. Obama has stressed that whatever action he takes, it will not result in placing American troops on the ground in Syria.

The simple case for action is the administration's contention that the sarin gas attack violated not only the international standard against using such weapons but also crossed Obama's "red line," set more than a year ago, that such use of weapons of mass destruction wouldn't be tolerated.

Intervening in Syria's conflict is no light matter, however. The fight has claimed more than 100,000 lives in the past 2½ years, and it has evolved from a government crackdown on a largely peaceful protest movement into a full-scale civil war scarily reminiscent of the one that ravaged Iraq over the last decade. Ethnic massacres have been committed by both sides, which both employ terrorist organizations as allies.

Obama faces a Congress faces a Congress divided over an unavoidably tough vote-of-conscience on overseas conflict rather than the more customary partisan fights over domestic policy.

Since Obama's stunning announcement Saturday that he'd seek congressional authority, dozens of members of Congress have issued statements. Most have praised the administration for seeking congressional authorization, and several have suggested they are leaning one way or another. But precious few have come out definitively one way or another.

McCain said he believed many members were still "up for grabs."