Opposition to U.S. military strikes against Syria may be building among Democrats' liberal rank and file as Congress opens debate over whether it should authorize those attacks.

The Progressive Change Campaign Committee on Wednesday announced its opposition to President Obama’s plan to hit Syria in retaliation for President Bashar Assad’s Aug. 21 chemical weapons attack that left more than 1,400 Syrians dead, including 400 children.

The PCCC regularly directs its advocacy toward Democratic members of Congress, and its announcement on Syria allies it with other liberal voices, including MoveOn.org and Markos Moulitsas, founder of the influential liberal website Daily Kos.

“Your progressive base stands firmly against military action in Syria," the PCCC advised lawmakers in a memo.

Moulitsas expressed his opposition Tuesday in a blog post headlined, “Ratcheting up the Syria Hysteria."

“Politics aside, the policy of the matter remains as hopelessly wrong,” Moulitsas wrote. “The issue here is whether the use of American force can improve a bad situation. And until someone makes an argument that rises above 'Assad is HITLER!' or 'we must do SOMETHING!,' fact is that engaging in Syria is a bad call. And so far, all we're seeing is a lot of table pounding.”

It's unclear, however, how much pressure anti-war progressives can bring to bear on House and Senate Democrats, who are more sensitive to grassroots pressure than Obama, given that the president is not on the ballot in 2014.

Top congressional Democrats, including House Minority Leader Nancy Pelosi, of California, and Senate Majority Leader Harry Reid, of Nevada, are strongly backing Obama’s Syria resolution.

Greg Sargent, author of the Washington Post’s Plum Line blog, which monitors Democratic politics, wrote Wednesday that anti-war liberals were likely to have a hard time organizing a campaign that could put the brakes on Obama’s drive to use military force against Syria.

“Multiple other conditions are conspiring to create challenges for anti-war activists that in some ways are more difficult than the ones they faced in the case of Iraq,” Sargent said.