WASHINGTON, D.C. ? President George W. Bush announced the resignation of his chief of staff today after a sharp decline in his approval ratings and a campaign by Republicans for a shake-up at the White House.

Bush replaced Andy Card with budget director Joshua Bolten, who has extensive experience both in Congress as well as the White House. Both Card and Bolten also served on the White House staff of Bush’s father, President George H.W. Bush.

“Josh is a creative policy thinker,” Bush said. “He is an expert on the budget and our economy. He is a man of candor and humor and directness. No person is better prepared for this important position.”

The move came as Bush is buffeted by increasing criticism of the drawn-out war in Iraq and as fellow Republicans have suggested pointedly that the president bring in new aides with fresh ideas and new energy.

A recent AP-Ipsos Poll found that Bush’s job approval had dipped to 37 percent, his lowest rating in that poll. Republicans fear the low ratings will hurt them in congressional elections in November and possibly even cost them control of both chambers of Congress.

Bush has been fiercely loyal to his aides, and changes in the top ranks of the White House have been rare. He announced the changes in a nationally broadcast appearance in the Oval Office.

A Democratic critic of Bush, Sen. Charles Schumer, said the Bush administration “finally realized it needs to change its ways, but the problems go far deeper than one staffer.”

There was no immediate indication of other changes afoot, but the White House did not close the door on a broader staff reorganization. White House press secretary Scott McClellan said Bolten will have the authority to make personnel shifts that he deems necessary. Additionally, McClellan would not say whether top aides, such as the two current deputy chiefs of staff, Joe Hagin and Karl Rove, would remain in place.

“All of us serve at the pleasure of the president,” McClellan said. “It’s premature to talk about any future decisions that may or may not be made.”

Bush called Bolten a man with broad experience, both in the financial world and in Washington, including the last three years as director of the Office of Management and Budget. He was deputy chief of staff before becoming director of the Office of Management and Budget.

Card, a veteran of the administration of President Ronald Reagan as well as Bush’s father, was known for keeping his cool under pressure. He was the aide who calmly walked into a Florida school room and whispered into Bush’s ear that America was under attack on Sept. 11, 2001.

When the first President Bush got sick at a banquet in Tokyo, aides and security officials ran toward the president. Card ran in the opposite direction, out the door to make sure the motorcade was ready to rush Bush away.

Card stood stoically with his hands by his sides Tuesday as Bush praised his years of service through the Sept. 11 attacks, war and legislative and economic challenges. Gripping the podium, Card said in his farewell: “You’re a good man, Mr. President.” Card’s eyes were watery. Card said he looks forward to just being Bush’s friend. Bush then gave him five quick slaps on the back and the two walked out of the Oval Office together.

The president said Card came to him in early March and suggested he should step down from the job he has held from the first day of Bush’s presidency. Bush said he decided during a weekend stay at the presidential retreat in Camp David, Maryland, to accept Card’s resignation and to name Bolten as his replacement.

At a White House news conference last week, Bush was asked about rumors that a shake-up in the White House staff was imminent. Bush said he was “satisfied with the people I’ve surrounded myself with.”

Card was widely respected by his colleagues in the Bush White House. They fondly called him “chief.”

He usually arrived at work in the West Wing by around 5:30 a.m. and frequently did not leave until 9 or 10 p.m.

Card plans to stay on the job until April 14, when the switch with Bolten takes place.

Associates said that Card had wanted to establish himself as the longest serving White House chief of staff. Sherman Adams, who was President Dwight D. Eisenhower’s chief of staff, had that distinction. Card will miss that distinction by about seven months.