Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, is privately requesting between 30,000 and 40,000 more troops, a request that has produced "sticker shock" and "huge resistance" among key lawmakers, sources told FOX News.

Congressional liberals who led the charge against the Iraq war are starting to turn their attention to Afghanistan, putting pressure on the Obama administration to scale back even as it prepares to consider a likely request to increase the U.S. troop presence. 

Members of the "Out of Iraq" caucus are organizing into a new group whose mission will be to question the military surge in the country President Obama has deemed critical to the fight against terrorism. 

"He will hear from us," said Rep. Lynn Woolsey, D-Calif., who was a founding member of the Iraq caucus. 

"Afghanistan is not Iraq. The terrain is different. It's difficult. It's harsh. There's a question whether you could ever end all of your efforts successfully," said Rep. Sheila Jackson Lee, D-Texas, who is part of both groups. 

The resistance to the war is forming from inside the president's own party. In recent days, a number of prominent Democrats have questioned calls to increase the U.S. troop presence in Afghanistan. 

This could put Obama squarely between his party and his top military advisers and officials. 

Though Obama has already sent 21,000 troops and trainers to the country, an official request is widely expected for more U.S. troops beyond the 68,000 that will be there by the end of the year. 

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told a Senate panel Tuesday that the war effort requires another 4,000 U.S. troops to train the Afghan army and an unspecified number of additional U.S. combat forces to defeat the Taliban and Al Qaeda. 

Obama on Wednesday said there is no "immediate decision pending" on more troops to Afghanistan. 

Asked if U.S. and NATO forces are winning the war, the president offered no direct response. 

"My determination is to get this right," he said. "You don't make determinations about resources, and certainly you don't make determinations about sending young men and women into battle, without having absolute clarity about what the strategy's going to be." 

Meanwhile, Obama aides dismissed questions about the mounting opposition to the war from within the Democratic Party. 

"The president is looking at this not through a political lens, but how do we get a very important national security concern right?" White House Press Secretary Robert Gibbs said. 

FOX News' James Rosen contributed to this report.