WASHINGTON — President Bush, defending an unpopular war, ordered gradual reductions in U.S. forces in Iraq on Thursday night and said, “The more successful we are, the more American troops can return home.”

Yet, Bush firmly rejected calls to end the war, insisting that Iraq will still need military, economic and political support from Washington after his presidency ends.

Bush said that 5,700 U.S. forces would be home by Christmas and that four brigades — for a total of at least 21,500 troops — would return by July, along with an undetermined number of support forces. Now at its highest level of the war, the U.S. troop strength stands at 168,000.

“The principle guiding my decisions on troop levels in Iraq is: return on success,” the president said, trying to summon the nation’s resolve once again to help Iraq “defeat those who threaten its future and also threaten ours.”

With no dramatic change in course, Bush’s decision sets the stage for a fiery political debate in Congress and on the 2008 presidential campaign trail. Democrats said Bush’s modest approach was unacceptable.

“An endless and unlimited military presence in Iraq is not an option,” said Sen. Jack Reed of Rhode Island, a former Army Ranger, who delivered the Democratic response.

“Democrats and Republicans in Congress — and throughout the nation — cannot and must not stand idly by while our interests throughout the world are undermined and our armed forces are stretched toward the breaking point,” Reed said. “We intend to exercise our constitutional duty and profoundly change our military’s involvement in Iraq.”

The reductions announced by Bush represented only a slight hastening of the originally scheduled end of the troop increase that Bush announced in January. When the cutbacks are complete, about 132,000 U.S. forces will be in Iraq.

Bush described the withdrawals, and the U.S. forces still fighting in Iraq, as a compromise on which war supporters and opponents could agree.

“The way forward I have described tonight makes it possible, for the first time in years, for people who have been on opposite sides of this difficult debate to come together,” Bush said.

That appeared highly unlikely, however, based on the reaction of Democratic leaders who want deadlines for withdrawals.

Majority Democrats in Congress are unable to muster enough votes to force an end to the war. So they are hoping to win Republican support with legislation to limit the mission of U.S. forces to training Iraq’s military and police, protecting U.S. assets and fighting terrorists.