"Secretary Rumsfeld's war plans in Iraq have failed. The country is on a dangerous course, and the administration has finally recognised the need for drastic, immediate change," said US Senator John Rockefeller, top Democrat on the Senate Intelligence Committee.
"The next Secretary of Defence will play a critical role in changing the course in Iraq, but will also have an enormous responsibility in addressing the global war on terror," Rockefeller said.
"Democrats stand ready to work with this Administration and its new leadership to begin to chart the right course."
Rumsfeld stood down on Wednesday and President George W. Bush nominated former CIA director Robert Gates to take his place, one day after Democrats seized the majority in the House of Representatives that had been held by Republicans since 1994.
Rumsfeld was a regular, if reluctant witness on Capitol Hill since the start of the Iraq war three and a half years ago, and has been a lightning rod for US policy there, amid numerous calls for his ouster.
"Secretary Rumsfeld is exhibit 'A' of the president's failed Iraq policy," said Democratic Representative Ed Markey, a long-time critic of the Bush administration's policy on the war.
The liberal Massachusetts lawmaker lamented that Rumsfeld's departure apparently would deprive Democrats of the chance to hold him to account with their increased power in the House of Representatives and Senate.
"It now appears that he has decided to 'cut and run' instead of facing a new Congress to whom the administration must answer on what went wrong in Iraq and how we can now change direction," Markey said.
Senator Chuck Schumer, the architect of big Democratic gains in the Senate in Tuesday's midterm balloting, said the election outcome was "a cry for change."
"For the first time it looks like the president is listening," he said, adding that "the nomination of a new Department of Defence secretary is a good first step, and we hope it is a sign that the president is looking toward a new course of action in Iraq."
"We look forward to working with him on an Iraq policy that is both strong and smart."
Another outspoken Rumsfeld critic, Democratic Senator Chris Dodd, said: "Voters across the country have made it clear that they want a new direction for our country at home and abroad, and a new tone in Washington. It's time for the errors and rancor of the past few years to end, as well."
He added: "We need to come together in a spirit of co-operation and collegiality and begin again to move our country forward. There is no time to waste."
The Republican leader in the Senate, Bill Frist, said: "I hope that this change will bring a fresh perspective to our fight on the global war on terror and help build a bipartisan defence policy in the months ahead.
"Congratulations to Robert Gates on his nomination and I look forward to his swift confirmation," Frist added.
Meanwhile, an outspoken veterans' group said they doubted the change at the Pentagon would yield real reform.
"Unless the president commits to changing the course in Iraq and developing a real victory strategy that will lead to an exit from Iraq, this doesn't even matter. It will be nothing more than a cosmetic change," said the VoteVets Action Fund, in a statement.

Copyright AFP (Agence France-Presse), 2006