Others say the president hasn't explained why intervening is in America's interest.
After a Labor Day weekend spent listening to concerned constituents, Sen. Chris Coons, D-Del., said the administration needed 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."
(Read more: Syria's chemical weapons aren't a simple target)
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.
In a post on his website, Rep. Rick Nolan, D-Minn., 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 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 and then attending an economic summit in St. Petersburg, Russia.
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 Obama's "red line," set more than a year ago, that such WMD use wouldn't be tolerated.
The U.S. said the attack killed 1,429 people, including at least 426 children. Those numbers are significantly higher than the death toll of 355 provided by the aid group Doctors Without Borders or the 502 confirmed dead by the Britain-based Syrian Observatory for Human Rights.
(Read more: What a Syria strike would look like)
Intervening in Syria's conflict is no light matter, however. Having claimed more than 100,000 lives in the past 2½ years, the fight 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 each employ terrorist organizations as allies.
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 its course of action, 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."
—By The Associated Press.