The morning after his reelection by a wide margin in 1972, President Richard M. Nixon summoned his Cabinet and top aides to the White House's Roosevelt Room and told them all that they had to resign.

Nixon, who kept only five Cabinet members, later said his goal was not a personnel purge but merely to start his second term with a symbolic clean slate. Morale plummeted among his most loyal subordinates, however, and Nixon in his memoirs called the move "a mistake."

President Bush -- indeed, any other president -- is unlikely to repeat that error as he and his staff carry out the seasonal review of Cabinet members and other key personnel that accompanies the transition to the second four years of a presidency, said G. Calvin Mackenzie, professor of government at Colby College in Maine and an expert on presidential transitions.

"The reaction was enormously negative," Mackenzie said of Nixon's maneuver. "Virtually every Cabinet officer who wrote a memoir underscored that as the worst moment in their public service."

With a contentious election behind them, Bush and his aides are now turning their attention to a transition process that probably began weeks ago and will undoubtedly involve departures, additions and other changes in the ranks of political appointees for months to come.

The White House did not respond to inquiries yesterday about this year's process, but analysts and participants in past presidential transitions say moving from a first term to a second is not as hectic or difficult as building a new administration from scratch. Still, there probably will be substantial turnover as some Cabinet members are replaced or reshuffled, and various deputies and assistants weigh whether to stay put, jockey for a better job or head to the private sector.

"There's more of a transition there than meets the eye" in a second term, said Paul C. Light, a government scholar at New York University who has studied the political appointee process. "There's going to be a fair amount of turnover. I'd estimate that roughly half of the Bush Cabinet and sub-Cabinet will be departing over the next six months, another 30 percent will change jobs and another 20 percent will stay put. That's just a rough guess based on past patterns."

Some long-serving political appointees may opt to return to the private sector to sit out a one-year federal lobbying ban so they can cash in on their contacts while Bush is still in office, he said.

No changes have been announced, but Secretary of State Colin L. Powell and Transportation Secretary Norman Y. Mineta, a Democrat who also served in President Bill Clinton's Cabinet, are widely thought to be contemplating moving on. Light said he expects lower-level departures at the Justice Department and the Department of Homeland Security, where the demands of the war on terrorism and, in DHS's case, cobbling together a new agency, have made difficult jobs even more exhausting.

Leon E. Panetta, who served as White House chief of staff during Clinton's 1996 transition, said it is likely that Bush officials have already approached Cabinet members and other key officials about who plans to leave, and developed lists of potential successors for the president to consider.

"By this point they should have a pretty good idea as to which Cabinet members are going to resign and which ones will stay on. And then, of course, there are always those they may not want to stay on -- and they'll have to deal with that," Panetta said, chuckling. "You really ought to . . . try to complete the process by the end of the year so you've got a team in place to begin to continue the administration in the new year."

Panetta said personnel decisions turn on many considerations, including the need to avoid burnout among appointees, ensure that federal agencies stay on course, reward loyalists and bring in fresh talent.

"I don't think these jobs ought to be career positions," Panetta said. "Sometimes you really help the president by virtue of moving on and letting some new blood into the operation."

Mackenzie said presidents face less pressure to repay political debts in choosing appointees during a second term. Bush "has more latitude than he had four years ago to choose people who exactly fit his needs as he unfolds a new policy agenda," Mackenzie said.

The scramble and positioning for administration jobs will soon kick into high gear. The "Plum Book," the guide to the executive branch's political jobs published after every election, is expected to be released by the House Government Reform Committee by the middle of next month, a committee spokesman said.

Light said the jobs will not be as attractive this year as they were during Bush's first term. Lame-duck presidents tend to accomplish less in their final years than they do in their first term, he said.

"Historical patterns on a second term suggest that it's just not as exciting a place to be," Light said. "The reality is the second-term presidency has a very short time to make a difference, and then the conversation shifts to the battle for the heart and soul of the party in 2008."

Researcher Meg Smith contributed to this report.

From left, Homeland Security Secretary Tom Ridge, Defense Secretary Donald H. Rumsfeld, Health and Human Services Secretary Tommy G. Thompson, National Association of Manufacturers President John M. Engler and Secretary of State Colin L. Powell await President Bush's arrival at a victory ceremony at the Ronald Reagan Building.