McCain, the top Republican on the Senate Armed Services Committee, said in an interview with the Associated Press Friday he thinks the plan is "significantly different" from the one Mr. Obama pushed during his campaign.
Before the election, Mr. Obama had advocated a steady and complete withdrawal of all U.S. forces within 16 months of taking office.
McCain said that members of Congress were told in a White House meeting Thursday that the majority of troops now in Iraq would be kept there through the end of 2009 to protect against violence during elections to be held in December.
Then, even as troops begin to leave (target date: August 31, 2010), between 35,000 and 50,000 forces would be kept behind to advise the Iraqi military.
McCain said his understanding is that the troops remaining in Iraq to serve as advisors would still go on combat patrols alongside Iraqis.
"They'll still be in harm's way," he told the AP. "There's no doubt about it."
The last U.S. troops are to be out of Iraq no later than Dec. 31, 2011, the deadline set under an agreement the two countries sealed towards the end of George W. Bush's presidency.