House Appropriations Defense subcommittee Chairman John Murtha (D-Pa.) said Monday that he expects that Republican lawmakers will begin abandoning President Bush’s Iraq policy after the GOP picks a presidential candidate next year.

Murtha also predicted that, despite the unpopularity of Congress, Democrats will make broad gains in next year’s election because voters are upset with the war.

“People are frustrated, but you’re going to see a big Democratic increase,” he told reporters after his speech. “I think we’ll pick up 40 [to] 50 seats.”

The lawmaker, a key adviser to House Speaker Nancy Pelosi (D-Calif.) on military issues, is not waiting on the presidential calendar to push for a broad troop withdrawal from Iraq. Murtha said he expects the Iraq war supplemental spending bill, up for debate in October or November, to include a specific date for withdrawal.

During his hour-long presentation, Murtha said he was not familiar with legislation by Reps. Neil Abercrombie (D-Hawaii) and John Tanner (D-Tenn.) calling on President Bush to report on his withdrawal planning. Many centrists in both parties see the legislation as a vehicle for bipartisan action on Iraq in Congress.

Murtha also tersely said he would denounce the newspaper advertisement taken out by the liberal group MoveOn.org criticizing commanding Gen. David Petraeus as “General Betray Us.” Without elaborating, he said “Yes” when asked if he would distance himself from the ad.