The Republican revolt and the interim assessment on the success of the US troop surge could accelerate a Democratic-led effort to try to force Bush to start scaling back troop levels in Iraq.
Democratic leaders have said they expect to soon pass a bill requiring the withdrawal of US combat troops from Iraq by April 1, 2008.
A redeployment would begin within 120 days and the president would be forced to report to Congress on why soldiers should stay in Iraq for "limited purposes".
"Hope and reality"
Although Republican discontent is growing over Iraq, it is not clear if the Democrats have drawn enough former allies of Bush to clear the 60-vote hurdle in the 100-seat Senate which is needed for success.
Meanwhile, the political debate over the war took a new twist on Wednesday, as another Republican senator bowed to public opinion and declared she would back a Democratic bid to enforce troops withdrawals by next year.
Olympia Snowe, senator for Maine, said the US had arrived at a "crossroads of hope and reality" on the war and it was time to embrace "reality."
The White House, under political fire, has admitted unhappiness over the war had become the "central fact" of US politics, but rejected demands to reverse Bush's surge of 30,000 extra troops into Iraq.
Tony Snow, White House spokesman, said: "There's a lot of skepticism among Republicans ... they're getting an earful from constituents."
In a separate development, in a rare moment of unity on Iraq, the Senate voted unanimously to pass a measure censuring Iran for what it said was complicity in the killings of US soldiers.
Senator Joseph Lieberman, who framed the legislation, said: "Today's unanimous vote sends a strong, clear message from the entire Senate to the Iranians that we know what they are doing in Iraq, and they must stop."
Wednesday's amendment laid out what it said was evidence about proxy attacks by Iranian forces on US soldiers in Iraq and called for a regular US government report to Congress on Tehran's role in Iraq.
A USA Today/Gallup poll this week showed more than seven in 10 Americans favour withdrawing nearly all US troops by April.