On Jan. 20, 2001, Bill Clinton will undergo a swift, irreversible transformation from being the most powerful man in the world to being . . . this other thing, this whatchmacallit, this oddity: the Politician Formerly Known as President William Jefferson Clinton.

He will become a 54-year-old former two-term president, far too young to be an emeritus statesman, but too lofty, too thoroughly dusted in the residue of power, to take a regular job. After you've been to the moon, you can't become an airline pilot.

Clinton--distracted last year by scandal and impeachment, and this year by the war in the Balkans--has only recently begun to think about his post-presidential life, according to his boyhood friend and former chief of staff Thomas "Mack" McLarty. He said Clinton raised the topic a few months ago as they flew to Arkansas for the dedication of the home where he was born as a historic site. Clinton was emotional, reflecting upon his childhood and thinking about his mother, who had died while he was in office.

He told McLarty, "I've got a lot of thoughts, and I haven't gotten them all sorted out." But he had one clear impulse. "I think I want to write my book"--his memoir of the presidency. He said he wanted to make that a serious endeavor, not a knockoff job.

Working on the book, and on his presidential library, ought to keep Clinton occupied for at least a couple of years, McLarty concluded. But the entire conversation, McLarty estimates, lasted only five minutes.

The people closest to the president have made rapid strides to create their own future. He could wake up in a year and a half and discover that his main occupation is being the spouse of the distinguished United States senator from the state of New York. The only certainty for Clinton is an actuarial one: No one has ever left the White House with so great a life expectancy.

"Many former presidents don't have the luxury of being 54 years old and in the prime of their lives," says his longtime friend Skip Rutherford, who is working on setting up Clinton's presidential library and a separate foundation in Little Rock. "A lot of people at age 54 are still climbing to the top. He's reached the pinnacle. He's been there twice. And so now is an exciting time to start a new career."

"I hope he goes out and makes some money," says his former campaign strategist and perennial guard dog, James Carville. Some critics may carp if Clinton pockets big checks for speeches--as they did when Ronald Reagan once received $2 million for a talk in Japan--but Carville says, "My advice is, to hell with them all, go ahead and take it."

What Clinton is unlikely to do is take any position that limits him, McLarty says. That's why he's unlikely to become, for example, a university president, forced to deal with salaries and tenure decisions and alumni backslapping and the latest political-correctness eruption on campus.

His friends don't pay much credence, either, to the recent report in the New Yorker magazine that Clinton has entertained the idea of running for the Senate from Arkansas in 2002. Clinton called that idea "crazy."

And there seems to be nothing but vapor behind the DreamWorks rumor, the notion that Clinton will go to California to be a movie mogul at the studio owned by his friends Steven Spielberg, David Geffen and Jeffrey Katzenberg. "One of you guys was sitting around bored and invented it," DreamWorks spokesman Andy Spahn says of the rumor.

There is one constant on his resume: an interest in race relations. Clinton wants that to be a focus of the William J. Clinton Presidential Foundation, of which Rutherford is president.

"One issue we've talked about a great deal is the issue of civil rights, and continuing the dialogue on race. It's particularly appropriate to do that from Little Rock because of Little Rock Central High School and the 1957 integration crisis there," Rutherford says.

The Clintons have retained a Washington lawyer, Robert Barnett, to represent them in their post-governmental dealings. "They're both focused on doing their respective jobs and will not turn to these subjects for well over a year," Barnett says.

Mrs. Clinton's decision about whether to run for the Senate "is taking a lot of her energy," says her spokeswoman, Marsha Berry. Whatever she decides, both she and her husband will almost certainly write books. The first lady has already received an offer of $5 million to write a book for publisher Judith Regan.

Presidents are not entirely cast adrift on the ice floes of private life. Clinton will still have some Secret Service protection. The government will provide him with a limited budget for an office and a staff, in addition to his regular pension. President Bush received, in fiscal 1999, about $400,000 from the government to run his office, according to Bush's chief of staff, Mike Dannenhauer.

A number of ex-presidents have chosen to write their memoirs, putter around their libraries and then slowly fade away.

Harry Truman lived for two decades as an ordinary citizen in Independence, Mo., working on his memoirs, living frugally, occasionally popping up with an ornery comment. One day, riding down a country road, he stopped and helped an elderly lady herd some wandering hogs back toward their pen. Truman later explained that he'd been a farmer longer than he'd been a president. He liked to lead folks on tours of his library, uttering a favorite joke: "My choice early in life was either to be a piano player in a whorehouse, or a politician. And, to tell the truth, there's hardly any difference."

Dwight Eisenhower, who had been among the most powerful men in the world even before he was president, did not easily make the transition to private life. He and his wife, Mamie, retired in 1961 to Gettysburg, Pa., but he grew quiet, distant, enervated.

The night Lyndon and Lady Bird Johnson returned to Texas, they took a walk around their ranch and came upon a pile of their luggage in the carport. No one would be carrying it for them. "The coach has turned back into a pumpkin. And the mice have all run away," Mrs. Johnson said with a laugh. LBJ was depressed for months, and wouldn't speak of his time in the White House. He let his hair grow long, over his collar. He picked up smoking again, despite warnings that it might kill him. He was not wanted at the next Democratic convention. In four years, almost to the day, he made the full transition from world leader to political nonperson to dead man.

Gerald Ford played golf.

George Bush let everyone know, when he left the presidency, that he planned to get heavily into the granddad business. He did just that, and occasionally jumped out of a plane. Now he's reemerged as the sire of a look-alike version of himself who may well reach the White House. Unique among ex-presidents, he hasn't even written a memoir.

Which brings up Richard Nixon, the champion of the reputation-burnishers. For 19 years he cranked out one book after another, about himself, about Watergate, about foreign affairs. His rehabilitation was never complete; Watergate remained firmly rooted in the first line of his obituary.

Ronald Reagan lives, but the Gipper is gone.

Finally there's Jimmy Carter, who has had the most active ex-presidency in recent times, traveling the globe to monitor elections and broker peace deals. Carter spent a few years after his 1980 defeat thinking that he might someday be elected again; when that faded, he reinvented himself as a freelancer.

The presidential historian Michael Beschloss ventures that Clinton will model himself to some degree after Carter, but will also engage in some serious Nixon-style reputation-burnishing. At some point, he'll have to address the Lewinsky scandal and his historic impeachment.

In other words, Clinton will still campaign. The pundits have credited Clinton with the invention of the "permanent campaign," a system in which the campaign-style politicking never ceases even after the race is won. Now Clinton has a chance to extend the campaign into his post-presidential career. The historians will have to write about a presidential administration even as the president in question continues to weigh in with his own opinions, kvetching and heckling from the sidelines.

"I'd be very surprised if he did not make a big effort in his ex-presidency to make a strong case for himself as having been a good president," Beschloss says.

Clinton may take a glance at another young ex-president, Teddy Roosevelt. In the closing days of his administration, he wrote a note to his successor:

"Ha! Ha! You are making up your Cabinet. I in a light-hearted way have spent the morning testing the rifles for my African trip. Life has compensations!" On his safari, he and his party bagged five elephants, seven hippos, nine lions and 13 rhinos. He later ran for president, one last time, as the candidate of the Bull Moose Party, famously delivering a speech even though minutes earlier he'd been shot in the chest by a would-be assassin ("I'll do the best I can, but there is a bullet in my body," he told the crowd).

A final fact: Bill Clinton can still drive a car, sources say.

It is not clear when Clinton was last behind the wheel. It is known that he owns a 1967 Mustang convertible, and that, once, while president, he was allowed to drive it at an auto show. That was five years ago.

A year and a half from now, the president can be a wheelman again.

"That could be a hazard to public safety," McLarty observes.

CAPTION: After leaving office, Lyndon Johnson let his hair grow over his collar.

CAPTION: Former President Jimmy Carter spends some of his scarce free time building houses for the poor.

CAPTION: Former President Richard Nixon (here in 1971) often strolled the beach near his San Clemente, Calif., home.