WASHINGTON — Democrats in Congress will begin on Wednesday the next phase in the battle for Iraq by unveiling the latest challenge to the military’s contention that the surge in Iraq has brought some success.

On September 5, the House Foreign Affairs Committee will hold a hearing to discuss a report from the Government Accountability Office that contends Iraqi security services have not improved and key political benchmarks have not been met. The report also says claims that violent attacks in Baghdad have decreased are disputed among different agencies, according to a draft that was reported on in yesterday’s Washington Post.

Congressional leaders were already touting the Washington Post article this week in anticipation of the testimony of the top American general in Iraq, David Petraeus, and of the American ambassador in Baghdad, Ryan Crocker. Their testimony is expected to accentuate the positives of the American military surge, a strategy that did not go into full effect until the end of June.

The Democrats commissioned the GAO study in the spring as a condition of the temporary bill to fund the wars in Afghanistan and Iraq, funding that will expire next month and whose future is in the hands of a Congress whose leaders have vowed to bring American troops home from what they say is a civil war in Iraq. President Bush has warned that if America retreats, the terrorists will follow us home. Unlike a White House assessment this summer, the GAO report measured not the progress Iraq’s military and government were making toward self-sufficiency and political reconciliation, but whether such key benchmarks were met.

General Petraeus is expected to stress that American troops have for the first time brought Sunni sheiks into the political process and found effective allies in the country against Al Qaeda, a benchmark that the GAO study does not measure. Already the State Department is making arrangements for an envoy of the Anbar Salvation Front, the umbrella group fighting Al Qaeda in that province, to come to Washington. The group’s leader, Abdul-Sattar al-Rishawi, is preparing his own message to Congress, according to one military official.

The draft of the GAO report finds that 13 out of 18 benchmarks included in the funding bill are not met. Yesterday, Pentagon officials confirmed that they would be asking the GAO to make some changes. A Pentagon spokesman, Geoff Morrell, told the Associated Press yesterday that the military had offered to make “factual corrections” to the draft. “We have provided the GAO with information which we believe will lead them to conclude that a few of the benchmark grades should be upgraded from ‘not met’ to ‘met,'” he said.

The Senate majority leader, Harold Reid, a Democrat of Nevada, seized on the GAO report yesterday. “The forthcoming GAO report offers a clear assessment that a new direction in Iraq must begin immediately, before more American lives are lost and more taxpayer dollars wasted. Democrats will continue to push for a new strategy to protect our troops and make America secure,” he said.

Mr. Reid yesterday went on to say he was concerned about reports that the White House was trying to “soften” its own assessment to Congress and that it may have taken out harsher assessments from the National Intelligence Estimate released this month. That estimate concluded that it was unlikely the current Iraqi national government would achieve political reconciliation among the warring Iraqi factions in the near or long term.

What the senator did not say, however, is that he would be pressing for legislation to defund the war. Nor did he in any way tip his hand with regard to his party’s strategy on a withdrawal vote.

The Democrats are in a tricky position. Groups like moveon.org and the Web site Daily Kos have in the last two weeks grown louder in their protests against Democrats who have returned from Iraq to report progress, at least on the military side. Moveon.org, for example, this week launched an ad against Rep. Brian Baird, a Democrat of Washington state, featuring an Iraq veteran saying it was immoral not to bring the soldiers home. This week, Mr. Baird, who voted against authorization for the war in 2003, wrote an op-ed for the Seattle Times in which he said the troops needed “more time to succeed.”

One Republican congressional aide who is working closely on the war votes said the best case scenario would be for a numeric majority in the House voting against any premature date for withdrawal. “There is no chance there are enough votes at this point in either the House or the Senate to force an end to the surge,” the aide said. “How are you going to tell constituents back home, ‘Well, the generals in Iraq say we are making progress now, so we should just pull the plug?'”

One measure General Petraeus is expected to tout as an indicator of the surge’s success is the decline in American casualties in recent months.

He is also expected to point out that the number of attacks in Baghdad from Al Qaeda has declined since the group has lost its base in Anbar. Instead, the group has resorted to attacking in less-protected villages.