WASHINGTON — A leading Democratic war critic said yesterday that he’ll soon push legislation that would order American troop withdrawals to begin in two months and predicted Republicans will swing behind it this time.

A vote on Rep. John Murtha’s proposal likely will come in September, when Iraq commander General David Petraeus delivers a long-anticipated assessment on the war and members of Congress weigh some $600 billion in defense spending requested by President Bush.

Under his plan, Mr. Murtha, a close ally of the House speaker, Rep. Nancy Pelosi, said he envisions troop withdrawals to start in November and take about a year to complete.

The House has passed similar proposals in the past, including one that Mr. Bush vetoed. But Mr. Murtha said he predicts this vote will be different because of mounting voter frustration with the war and a lack of progress in Iraq.

“This is big time,” Mr. Murtha of Pennsylvania told reporters of the upcoming war debate in September. “When you get to September, this is history. This is when we’re going to have a real confrontation with the president trying to work things out.”

Mr. Bush has not given any indication he is open to a dramatic shift or a major redeployment of troops after September. He instead has talked at length about the need to remain engaged in Iraq to fight Al Qaeda and has repeatedly appealed to lawmakers for more patience.

For their part, top American military officers also have indicated that the troop buildup initiated this year may be needed through next summer.

Congressional Republican leaders have been willing to support Mr. Bush so far, although they concede keeping their rank-and-file in line behind the president will become considerably tougher come September.

In response to Mr. Murtha’s proposal, the House Republican leader, Rep. John Boehner, said Democrats were ignoring progress in Iraq.

“If they are not listening to reports from our generals today, how does anyone believe they will make an honest and objective decision in September?” Mr. Boehner of Ohio said. “Our national security is not a political football, and Republicans aren’t going to treat it as such.”

Mr. Murtha counters that he thinks Mr. Bush and GOP members will have no choice. To maintain current troop levels through 2008, Mr. Murtha predicted that combat tours would have to be extended from 15 months to 18 months — a politically unsavory position to take on an already deeply unpopular war.

The Defense Department has said extending combat tours of troops would be a last resort.

Mr. Murtha said he will propose his measure as an amendment to the $460 billion annual military funding bill, which the House will begin debating next week and probably finish in September.