The White House is expected to present to Congress an interim report on the situation in Iraq, assessing the results of the major US troop "surge".
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The report is due to measure the progress being made by the Iraqi government in meeting key benchmarks.
With violence continuing, the findings are widely expected to be mixed, correspondents say.
And the Bush administration has cautioned that it is too early to assess the effects of the "surge".
The interim report - mandated by the opposition Democrats - comes amid growing pressure in Congress to change course in Iraq.
The House of Representatives will shortly begin debating the issue, while the Senate is expected to vote next week on amendments to the military budget, which could force cuts in troop numbers.
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On Wednesday, National Security Adviser Stephen Hadley visited Capitol Hill as two more Republicans joined the ranks of those calling for a phased troop withdrawal.
Mr Bush says he will again veto any bill passed by Congress that sets such a timetable.
The Iraqi government has also expressed concerns about an early withdrawal.
The report is expected to show that the Iraqi government has made only limited progress in meeting 18 political and military benchmarks set out by the US Congress when it approved continued funding for the US deployment.
Half the benchmarks - those assessing the performance of the Iraqi security forces - are likely to show there has been progress, officials say.
But on those assessing political reconciliation, the progress made by Prime Minister Nouri Maliki's government is expected to be deemed insufficient.
These include failure to pass a law on sharing Iraq's oil revenues and another on redressing the effects of the de-Baathification policy - the purge of supporters of former Iraqi leader Saddam Hussein.
On Tuesday, Mr Bush said he understood how hard the Iraq war was for the US.
But he added that Congress must give his surge plan time to work, and urged lawmakers to wait for the full report, due to be delivered in mid-September by the head of US forces in Iraq, General David Petraeus.
General Petraeus said earlier this week that it was too soon to tell if the surge was working and told the BBC that fighting the insurgency was a "long term endeavour" which could take decades.
US Democrats want a timetable for withdrawal or a change in the focus of the US mission from combat to counter-terrorism and the training of Iraqi forces.
Mr Bush has said he will veto a bill on a pull-out timetable
And some senators from Mr Bush's Republican party are now joining ranks with them.
"I'm hopeful they change their minds," Senator Pete Domenici said after Wednesday's meeting that senators had with Mr Hadley.
Senior White House officials have spent the week on Capitol Hill trying to limit the rebellion and prevent legislation that would force the withdrawal of American troops, says the BBC's James Coomarasamy in Washington.
It is not yet clear whether a mixed progress report will help or hinder them, our correspondent says.