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 leaves for a three-day trip to Europe on Tuesday night, visiting Stockholm, Sweden, 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.
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."