Rice faces confirmation hearings in the Senate, where Democrats said they will take the opportunity to grill her about some of the discredited elements of the White House's case for war against Iraq. But congressional leaders predicted she will be confirmed, probably before Bush begins his second term on Jan. 20.
Powell, who was something of a misfit in a conservative-dominated administration, gave Bush his resignation letter Friday and announced his departure Monday.
Rice spends more time with Bush than any aide except perhaps Chief of Staff Andrew H. Card Jr., and she has developed a friendship with the president and first lady Laura Bush that frequently takes her to Camp David on weekends. It was there, the weekend after the election, that they began to have concrete talks about her new job, aides said.
Rice, a Cleveland Browns fan, bonded with Bush through their love of sports. The president drew laughter when he said in the Roosevelt Room: "Condi's true ambition is beyond my power to grant. She would really like to be the commissioner of the National Football League."
Powell's departure and Rice's arrival have been greeted nervously at the State Department, and she used her brief acceptance remarks to try to reassure the agency's workers. She said she found it "humbling to imagine succeeding my dear friend and mentor Colin Powell."
"If I am confirmed by the Senate," she said, "I look forward to working with the great people of the Foreign Service and the Civil Service. And one of my highest priorities as secretary will be to ensure that they have all the tools necessary to carry American diplomacy forward in the 21st century."
Deputy Secretary of State Richard L. Armitage, often described as Powell's best friend, also resigned. Powell has said he will depart after his successor is confirmed.
White House officials would not say who would be nominated as Rice's deputy other than to say that it will not be the most widely rumored possibility -- John R. Bolton, a hard-line former American Enterprise Institute scholar who is undersecretary of state for arms control and international security.
Administration officials said one possibility is Arnold Kanter, who was undersecretary of state for political affairs under the president's father, from 1991 to 1993.
Bush aides said he will replace more than half of the 15 heads of executive departments. Today, he is to announce that his domestic policy adviser, Margaret Spellings, a veteran of his Texas governor's office, will succeed Roderick R. Paige as education secretary.
That will mean that all three of the new Cabinet secretaries Bush has named so far -- Rice, Spellings and White House counsel Alberto R. Gonzales as the new attorney general -- are White House staff members, extending his personal control of the Executive Branch.
White House communications director Dan Bartlett is likely to shed some daily duties and assume an expanded portfolio, perhaps as Bush's counselor, a title that was retired when Karen Hughes left in 2002. If he changed roles, his likely successor would be Nicolle Devenish, communications director for the Bush-Cheney campaign. White House press secretary Scott McClellan will stay in his job.