Reid said: "At a time when President Bush is asking our troops to shoulder a larger and unsustainable burden policing a civil war, his failed policies have left us increasingly isolated in Iraq and less secure here at home."
Edward Kennedy, a veteran Democratic senator, branded the British announcement a "stunning rejection" of Bush's strategy.
He said: "Now our country's strongest ally has rejected it.
"No matter how the White House tries to spin it, the British government has decided to split with president Bush and begin to move troops out of Iraq."
Joseph Biden, the Senate foreign relations committee chairman and Democratic presidential candidate, said that Bush should follow Blair's example and "start to draw down our troops from Iraq, not send more into the middle of a civil war".
"Has any of the Bush Iraq plans worked other than causing the worst destruction?"
Zaffar Zohair, Islamabad, Pakistan
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The US is currently undergoing a national debate on America's war in Iraq - driven by the new Democratic-controlled Congress - that is also increasingly defining the scope of the 2008 US presidential elections.
The House of Representatives last week managed to pass a non-binding resolution opposing Bush's surge of 21,500 extra troops into the country.
Bush administration representatives led by Dick Cheney, the US vice-president portrayed the move by Britain as a sign of that conditions were improving in Iraq.
"Now our country's strongest ally has rejected it"
Edward Kennedy, Democratic senator
Tony Snow, the White House spokesman, rejected suggestions the British announcement amounted to a timeline to withdraw forces from the country.
He said: "What you had is progress first and then the removal. This was not in response to any calendar that had been set by the parliament or by the prime minister."
Earlier, Cheney said the British move was a direct result of progress made in the British area of operations around the southern Iraqi city of Basra.
"Well, I look at it and see it is actually an affirmation that there are parts of Iraq where things are going pretty well," Cheney said.
Condoleezza Rice, US secretary of state, said from Berlin: "The coalition remains intact and in fact the British still have thousands of soldiers deployed in Iraq."