President Bush will offer Saddam Hussein an ultimatum Monday night -- step down or face war.
Bush was giving the Iraqi leader 48 hours to comply, administration officials said. The president, commander in chief of 250,000 U.S. troops poised at the borders of Iraq, planned to address the nation at 8 p.m. EST.
Karen Hughes, a former White House aide who was helping Bush prepare for the speech, said the president would set a deadline for the Iraqi leader to leave.
She added that the speech, expected to last about 15 minutes, would present "a summation of what we have tried to accomplish at the U.N. Security Council, what we have tried to accomplish peacefully and of the threat Saddam Hussein's weapons pose to the world."
She said Bush would express disappointment with the U.N.'s lack of action and would speak directly to the Iraqi people in one portion of his remarks.
An intense White House debate over whether to establish a timetable was settled hours before the president's speech. Senior administration officials, all of whom spoke on condition of anonymity, said Bush had decided on a 48-hour deadline.
"The diplomatic window has now been closed," White House spokesman Ari Fleischer declared Monday morning, just 12 hours after Bush's return from an Atlantic island summit with his allies from Britain and Spain.
A quick round of telephone calls Sunday night and Monday morning confirmed what aides said Bush had concluded before the summit: The allies' U.N. resolution was doomed to fail.
He ordered the measure withdrawn to avoid an embarrassing defeat, then gave the go-ahead for a long-planned ultimatum address.
Bush also planned to use the prime-time address to explain why he was on the brink of ordering U.S. troops into action without U.N. approval.
The American public, by a 2-1 margin, generally supports military action against Iraq to remove Saddam, a slight increase from recent weeks, according to a CNN-USA Today-Gallup poll out Monday. Opinion was evenly divided when people were asked about an attack without an attempt to gain U.N. backing.
Bush was expected to warn about risks facing U.S. troops and perhaps Americans at home, reflecting concern that terrorists will try to retaliate during the war.
The speech would serve as a warning to journalists and humanitarian workers to leave Iraq. "Baghdad is not a safe place to be," Fleischer said.
Bush also planned to outline his plans for helping Iraq recover from military conflict and become a democratic nation after U.S. occupation ends.
This was not the time for Bush to list which nations would join the U.S. in fighting Iraq, officials said. That would come only after the fighting had started, when the president would address the nation from the Oval Office, they said.
However, Bush asked Australia to participate in a "coalition of the willing" preparing for war against Iraq, Prime Minister John Howard said.
White House and congressional sources said Bush intends to send Congress a bill seeking more than $70 billion to pay for the war.
Seven months ago, Secretary of State Colin Powell helped persuade Bush to seek U.N. approval for military action despite the objections of anti-Saddam hawks like Vice President Dick Cheney.
His diplomacy derailed, Powell sounded ready to turn to war. "The moment of truth is arriving," said the retired Army general was chairman of the Joint Chiefs of Staff during the 1991 Persian Gulf War led by Bush's father.
He said the only way war could be avoided was for "Saddam Hussein and his immediate cohorts to leave the country."
Senior White House officials said they did not expect Saddam to seek exile. Thus, Bush planned to be at war within a matter of days, they said.
Powell suggested that even an 11th-hour effort by Saddam to disarm wouldn't avoid war.
"I can think of nothing Saddam Hussein could do diplomatically," he said. "He had his chance."
Fox News' Liza Porteus, Teri Schultz and Jim Angle and The Associated Press contributed to this report.