"A fixed timetable for withdrawal, in my judgment, means defeat," he said.
In a somber, pre-election review of a long and brutal war, Mr. Bush conceded that the United States is taking heavy casualties and said, "I know many Americans are not satisfied with the situation in Iraq."
"I'm not satisfied either," he said at a speech and question and answer session at the White House 13 days before Nov. 7 congressional elections in which Republican control of the House of Representatives and the Senate is at stake.
"Americans have no intention of taking sides in a sectarian struggle or standing in the crossfire between rival factions," Mr. Bush said.
Several Democratic critics have said that is precisely what the administration is risking with an open-ended commitment of American forces, at a time that a year-old Iraqi government gropes for a compromise that can satisfy Sunni, Shiite and Kurdish political interests.
Mr. Bush gave Iraq's Prime Minister Nuri al-Maliki a vote of confidence just hours after Mr. Maliki called his own news conference to reject timetables set on Tuesday for Iraqis to take over from U.S. troops, reports CBS News chief White House correspondent Jim Axelrod. Mr. Bush was cautiously supportive.
In Baghdad, the Iraqi leader took a hard slap at the United States for a raid by U.S. and Iraqi forces on the stronghold of a Shiite militia led by a radical anti-American cleric on whom Al-Maliki relies for political support. Al-Maliki said the raid "will not be repeated."
Al-Maliki also criticized the top U.S. military and diplomatic representatives in Iraq for saying his government needed to set a timetable to curb violence in the country. "I affirm that this government represents the will of the people and no one has the right to impose a timetable on it," he said.
It was just two weeks ago that the president had his last formal, solo White House news conference, and he's never had two of them this close together, reports CBS News White House correspondent Mark Knoller. But with the midterm elections less than two weeks off and control of Congress at stake, he wants his message to be heard.
In his opening moments at the podium in the East Room of the White House, Mr. Bush departed starkly from a practice of not talking about specific deaths in Iraq.
"There has been heavy fighting, many enemy fighters have been killed or captured and we've suffered casualties of our own," he said. "This month we've lost 93 American service members in Iraq, the most since October of 2005. During roughly the same period, more than 300 Iraqi security personnel have given their lives in battle. Iraqi civilians have suffered unspeakable violence at the hands of the terrorists, insurgents, illegal militias, armed groups and criminals."
He called these events "a serious concern to me, and a serious concern to the American people."