By Matthew Davis
BBC News, Washington
President Bush's chief-of-staff Andrew Card has quit after five years in one of the most-demanding and important jobs in the administration:
Mr Card has been seen as a less-powerful figure than some chiefs
As head of the 400-strong White house staff, Andrew Card was chief among a top tier of some 19 presidential advisers, all earning more than $160,000-a-year, many of whom have been with the president through the five-years of his presidency and his first election campaign.
They include figures like deputy chief-of-staff Karl Rove, national security adviser Stephen Hadley and counsellor to the president, Dan Bartlett - all men whose every waking moment is a struggle to keep on top of events at home and abroad.
The chief-of-staff's exact duties vary from administration to administration, but he - and so far it has always been a he - is often one of the president's closest political advisers and a close personal friend.
Traditionally the chief-of-staff is the "gatekeeper" to the president.
Beneath him are a gamut of assistants, special assistants and advisers who - tied to BlackBerrys and mobile phones - keep the wheels of the administration turning.
Among the less-heralded posts are those of speechwriter to the First Lady, director of fact-checking and director of the Gift Office.
During the Reagan presidency, chiefs-of-staff James Baker and Donald Regan were seen as quasi-prime ministerial figures, because that president's hands-off style of handling the business of government.
Under George W Bush, Mr Card has been seen as a less-powerful figure, because the current US president deals much more closely with his cabinet secretaries.
Nevertheless, for more than five years Mr Card has maintained the punishing schedule of the White House chief-of-staff: days beginning before dawn and ending close to midnight.
Only one man has served in the role longer: Sherman Adams, under President Eisenhower.
Yet where once talk was of Mr Card's tenacity and staying-power, the misfortunes of the Bush administration have led to mutterings about a staff exhausted by the rigours of the job - and a series of crises.
"Andy Card has served me and our country in historic times:," Mr Bush said, "on a terrible day when America
was attacked, during economic recession and recovery, through storms of unprecedented destructive power, in peace and in war.
"Andy has overseen legislative achievements on issues from education to Medicare. He helped confirm two
justices to the Supreme Court, including a new Chief Justice."
But Mr Card has also served at a time when the unpopularity of the war in Iraq, high energy prices and the disaster of hurricane Katrina have conspired to give Mr Bush the lowest poll ratings of any second term president.
In recent months, White House staffers have also had to fire-fight the incendiary political effects of an
Arab-owned firm's failed takeover of certain American ports, the arrest of a top Bush adviser on theft
charges and the errant shooting of Vice President Dick Cheney, who injured a hunting partner.
There has been pressure from Republicans in Congress to bring in fresh faces amid a perception that the administration has fumbled such issues.
Yet it is unclear whether Mr Card's departure - and that of Interior Secretary Gale Norton - signals the start of a wider shake-up of the White House staff.
There have been rumblings about an unofficial ambassador to Capitol Hill who might help repair
relations between the White House and senior lawmakers, particularly frayed in the wake of the Dubai Ports World fiasco.
But while Presidents Ronald Reagan and Bill Clinton each had four chiefs-of-staff during their time on office, Mr Bush has emphasised loyalty.
He surrounds himself with people he knows well, and Mr Card's replacement, Josh Bolten, is just such a figure, having served as budget director since 2003 and deputy chief-of-staff for policy before that.
For now Mr Bush appears to be sticking with his tried and trusted formula.