Condoleezza Rice (searchwas "looking at notes and reviewing" documents Wednesday as she prepared to publicly testify the next day before the independent commission investigating the Sept. 11, 2001 attacks, a senior Bush administration official told Fox News.

"She's already spent more than four hours with the commission. She's prepared," the official said.

As of Tuesday night, aides were still reviewing drafts of the national security adviser's opening statement, which the source confirmed would run roughly 20 minutes in length.

Rice "views this as an important opportunity to work with the commission," the source said.

Asked if Rice had chuckled over suggestions her testimony could make or break President Bush's re-election, the aide replied, "She's taking it very seriously. This is no laughing matter."

Rice's much-anticipated appearance comes after the testimony of several high-ranking Bush and Clinton administration officials.

The panel, officially known as the National Commission on Terrorist Attacks Upon the United States (search), is investigating the military and intelligence issues surrounding the plane hijackings and crashes that left about 3,000 people dead.

The White House came under heavy fire when, citing the constitutional separation of the executive and legislative branches, it said Rice would not openly testify before the commission. White House lawyers argued her appearance could compel future presidential advisers to testify before Congress.

After criticism from commission members, the news media and congressmen of both political parties, the Bush administration decided to allow her testimony on condition that its acquiescence did not set a precedent.

Rice had previously met privately with the commission, but after the testimony of former Bush and Clinton counterterrorism adviser Richard A. Clarke (search), who claimed the Bush White House did not take Al Qaeda seriously before Sept. 11, panelists repeated their calls to hear publicly from Rice.

"I think she'll do a very good job," Sen. George Allen, R-Va., told Fox News on Wednesday. "She's an outstanding spokesperson not only for the Bush administration but for the American people."

Commission member Rep. Tim Roemer, D-Ind., told Fox News on Wednesday that the commission will also ask Rice why it took the new Bush team seven months to put together a counterterrorism strategy -- which was delivered to Bush's desk just days before the Sept. 11 attacks -- and how different that plan was from Clinton's.

"I think it's important this not turn into a media circus, that we not make this only a 'he said, she said,'" Roemer said. "We need to make this a serious, substantive, nonpartisan look at what was done right, what priority was put on terrorism, what was done wrong ... how the Bush administration dealt with both the short-term opportunities to do something about Al Qaeda."

Commission Chairman Thomas H. Kean said earlier in the week that the panel's report -- due out this summer -- will say, among other things, that the attacks were preventable.

"I think there's a difference between saying they were preventable -- which I'm not sure we can make that conclusion quite yet … you can say at this point, that there were failures, there was a national failure" in both administrations, Roemer said.

Rice to 'Lay Out the Facts'

Rice's opening statement will be a "detailed, almost day-by-day overview" of what the administration was doing in the months before the attacks, a White House official said.

"She's a very smart, capable person who knows exactly what took place and will lay out the facts," Bush said Tuesday.

Rice is expected to speak directly to survivors of those killed in the attacks. Some Washington insiders say the tone of her testimony must be measured, deliberate and noncombative but with some passion.

"It's about defending the president ... not discrediting opponents," presidential adviser David Gergen told the Los Angeles Times.

Rice has already been criticized for focusing on missile defense and not Al Qaeda in a speech she had planned to deliver on Sept. 11, 2001.

But President Clinton also made national missile defense a priority in his final counterterrorism report, which said the United States must confront new threats that included weapons of mass destruction, terrorism and nuclear weapons.

While Clinton's report, submitted to Congress in December 2000, doesn't mention Al Qaeda, it does name Usama bin Laden as leader of the terror network that carried out attacks on American embassies and planned more attacks.

"Our strategy pressures terrorists, deters attacks, and responds forcefully to terrorist acts," states the Clinton report. "It combines law enforcement and intelligence efforts; vigorous diplomacy, economic sanctions ... and when necessary, military force."

Clarke testified that the Clinton administration considered Al Qaeda an "urgent" threat. But the Clinton report characterizes terrorist attacks against Americans as "crimes" rather than an urgent threat. The Sept. 11 commission will examine these discrepancies.

"Obviously, a lot of these are issues that the 9/11 commission is looking at now, as they work to complete their report," said White House spokesman Scott McClellan. "And they're looking back not only at the eight months when this administration was in office prior to Sept.11, but the eight years prior to that as well, when these threats were building and emerging."

Next week, two days of public hearings will include testimony from Clinton Attorney General Janet Reno, CIA Director George Tenet, FBI Director Robert Mueller, former FBI Director Louis Freeh and Attorney General John Ashcroft.

Fox News' Liza Porteus, James Rosen and Kelly Wright contributed to this report.