"I'm ready to move on," McClellan said while standing next to President Bush on the South Lawn of the White House.
McClellan, who has served as Bush's spokesman and defender during an especially trying time in the presidency, indicated that his decision was part of an ongoing personnel shake-up in the administration.
"The White House is going through a period of transition; change can be helpful, and this is a good time and good position to help bring about change," he said.
Bush said McClellan had performed a "job well done," and the two touched on their long relationship predating the presidency.
"I thought he handled his assignment with class, integrity," the president said. "It's going to be hard to replace Scott, but nevertheless he made the decision and I accepted it. One of these days, he and I are going to be rocking in chairs in Texas and talking about the good old days."
In the past year, McClellan has fended off aggressive questioning about the federal investigation of the leak of a CIA officer's name, the aftermath of Hurricane Katrina and Vice President Dick Cheney's accidental shooting of a hunting companion. Former top Cheney aide I. Lewis "Scooter" Libby has been indicted in the CIA leak probe, and Rove remains under investigation in the case.
While McClellan was adept at maintaining his cool during contentious and even hostile briefings, ill feelings seemed to boil over after the Cheney shooting incident. In an exchange not meant for the airwaves, NBC correspondent David Gregory snapped at McClellan. The reporter later apologized for losing his temper.
McClellan, who has served in the job since replacing Ari Fleischer in June 2003, plans to leave in two or three weeks.
Among those being considered to replace McClellan is FOX News Radio host Tony Snow, who once served as a speechwriter for President George H.W. Bush.
Others who have been approached about the position include former Pentagon spokeswoman Victoria Clark and Dan Senor, the former Coalition Provisional Authority spokesman in Iraq. Senor, now a contributor on FOX News Channel, served the U.S. civil administrator in Iraq, Paul Bremer.
Bush and McClellan boarded Marine One for a trip to Alabama after making the announcement. The president and his staff later traveled by motorcade to Andrews Air Force Base in Maryland, where they took Air Force One after a problem with Marine One's helicopter radio.
McClellan visited with reporters in the press cabin of Air Force One and shook reporters' hands. McClellan responded to a comment that it was a sad moment, saying, "It is sad on some level."
Rove Returns to Familiar Role
Rove is giving up oversight of policy development to focus more on politics as the fall midterm elections approach.
Just over a year ago, Rove was promoted to deputy chief of staff in charge of most White House policy coordination. That new portfolio came on top of his title as senior adviser and role of chief policy aide to Bush.
The job of deputy chief of staff for policy is being given to Joel Kaplan, now the White House's deputy budget director. The promotion of Kaplan would leave Bush with three deputy chiefs of staff: Rove, Kaplan and Joe Hagin, who oversees administrative matters, intelligence and other national security issues.
"It struck me as unusual when he was moved into a policy role in the first place," said Matthew Wilson, a political scientist at Southern Methodist University in Texas. "It took him out of his natural environment."
Rove is widely regarded as a brilliant political strategist, and Republican incumbents running for re-election this November may be glad to have his full attention again. With Bush's approval ratings at the lowest of his presidency, the GOP stands to lose key seats in Pennsylvania, Montana and Ohio. Bush himself may be growing more concerned with ending his presidency on a higher note.
"Some have pointed to Rove's transition to policy as the point at which the political operation at the White House became less smooth," Wilson said. "His policy duties may have created conflicts both in terms of time and focus from what he's really good at, which is selling other people's policy proposals. This move is an effort to refocus his energy and attention to where he can do the most good."
Some Republican lawmakers advocated firing Rove after Katrina, in part because they feared he was a political liability. But it is far from certain that Rove — who insiders say means more to Bush than the sum of his job titles — will no longer have any say in policy issues.
"A lot of these moves seem to me to be pretty much in keeping with moving chairs around the deck," said Bruce Buchanan, a political scientist at the University of Texas at Austin. "The title kind of ratified what he was already doing, and that was making a connection for the president between politics and policy. He has always done that."
Democratic critics did not seem moved by the change in job title. Howard Dean, chairman of the Democratic National Committee, accused Bush of "merely engaging in window dressing."
Dean was not alone in his belief that Rove's function would remain largely unchanged despite the reassignment.
"I have a hunch Rove will continue to have as much input on policy as Bush wants him to have," said H.W. Brands, presidential historian and author of "Andrew Jackson, Lone Star Nation" and "Woodrow Wilson."
"It may be a symbolic thing. The president has heard calls from within the Republican Party and the Democrats that there has to be a shake-up."
Regardless of how effective Rove can be in the 2006 and 2008 elections, how Republicans — and Bush's legacy — fare may be most dependent on the progression of the war in Iraq.
Six ex-generals this month called for Defense Secretary Donald Rumsfeld to resign. Though the generals' revolt is just the latest in what has been a steady drumbeat from critics calling for new oversight of the war in Iraq, Bush has been unwavering in his support for Rumsfeld.
Bush himself has acknowledged that Iraq will remain unstable well after he leaves office. In that event, Brands said, it was difficult to see how Bush could cram in a major legislative victory before he leaves office, no matter how capable Kaplan turns out to be.
"The two big domestic issues that aren’t going away are Social Security and health care. I would be astonished if anything happens on those in the last two years of any president's second term," Brands said.
FOX News' Wendell Goler and The Associated Press contributed to this report.