MoveOn.org, the liberal activist group that helped mobilize crucial support for President Obama during his 2008 presidential bid, launched a major effort Wednesday night to defeat the congressional resolution authorizing the use of military force against Syria.

The group, which arose out of Democrats' fierce opposition to the war in Iraq, made its decision after holding a virtual teach-in, monitoring petitions posted on its site and conducting a 24-hour poll that found 73 percent of its members oppose military intervention. Its ability to mobilize the Democratic Party's liberal base could complicate the White House's effort to win some of the House Democrats it needs to pass the resolution next week.