Jeff Greenfield had been on the job only two days, a "junior-grade assistant" fresh out of law school in the Senate office of Robert F. Kennedy, when an unexpected assignment plunged him into the "intensely exciting" world of politics.

Kennedy needed a speech on Vietnam, but "everybody was out of the office" that day. "Can you write a speech?" he was asked.

"Why not?" he thought, and immediately answered, "Yes."

The speech "went through many drafts," but Kennedy liked it. In that first week, Greenfield jumped from raw recruit to major speechwriter.

To Greenfield, his experience illustrates the "lures of politics" -- it's a "meritocracy" where "ability can be recognized almost immediately." It also points out a "time-tested" route to making a name for yourself in an electoral campaign:

"Get your foot in the door any way you can. Once you are inside a political arena, anything can happen."

In his case, "Being thrown from nowhere into a presidential campaign at 24 was very strange -- and very satisfying." He had become the youngest of Kennedy's famous 1968 Kiddie Corps.

"It sounds like Horatio Alger," says Greenfield, now 37 and an author who covers politics and the media for CBS-TV's "Sunday Morning," but "But I've seen it with my own eyes." When there's only "three weeks to go in a campaign," situations can get "desperate. If you have the ability to get things done -- and you're thrown an impossible task and you do it -- you'll be remembered."

After Kennedy's death, Greenfield became chief speechwriter for former New York Mayor John Lindsay and then spent seven years as a consultant with the political media firm of Garth Associates.

There, he says, he worked in about 30 campaigns, with both winners -- Gov. Hugh Carey of New York -- and losers -- former Sen. John Tunney of California.

"Sometimes the magic works and sometimes it doesn't," he says, quoting the words of the old Indian chief in the Dustin Hoffman movie "Little Big Man."

He has just written a primer in this presidential year for Americans seeking electoral office -- from town clerk to U.S. president -- or for those who want to work on a campaign staff. Its title: "Playing to Win: An Insider's Guide to Politics" (Simon and Schuster, 286 pages, $11.95).

For Greenfield, politics provides the excitement of a Broadway opening: the "dramatic" urgency, the "electric current," the sense of "everybody playing above his head."

For the winning candidate, there's the opportunity to make positive changes in our society. And the "young activist" on his or her staff, "can wind up in the White House at $56,000 a year."

Nevertheless, Greenfield has never felt the urge himself to enter a race because of the tremendous "personal costs."

"It isn't that I don't think the profession is worth it; I just think you can be a good political figure and a good father and husband."

A senator, he says once told him "how he had missed his daughter growing up. That's the loss."

And the "psychological burden" can be "tremendous" for a losing candidate who has stood before the electorate and received a resounding "no" in response. "I've seen candidates takes a long time to recover."

As for the candidate's staff, they spend summer weekends holed up in a "hot, fetid office eating Danishes and drinking cold coffee." They have an indoor pallor and are "30 pounds overwight" from weeks of hamburgers and French fries.

"You've really got to want that guy to win a lot," he says, "to endure all that."

Still "thousands of otherwise rational and intelligent people" plunge into the whirl -- in part, he believes, because "political life offers more chances to fulfill more desires than does almost any other enterprise."

The easiest -- and perhaps most obvious -- road to political success is to "have a great deal of money;" to "know people with a great deal of money;" to "attend an elite university;" and to "know someone well-placed in politics."

Harder -- but certainly not impossible -- alternate routes:

Becoming well-known in another field: You can "then demonstrate your capability," says Greenfield. "John Glenn, the astronaut who became Ohio senator, is an example."

Working your way up: "It's still true. You can go from school board to a local legislative seat to higher office. Most of the time you have to pay some kind of dues."

(Greenfield anticipates more women showing up "soon" on the national political scene because so many are serving in state legislatures.)

Taking part in a local issue: "It gives you credibility. Sooner or later you'll be invited to appear on a local TV program. Do that. People will learn you have something to say."

It helps to have the "Charm School" look of a Sen. Charles Percey (R-Ill.), but it's not necessary. "Tunney," with his Hollywood face, says Greenfield, "ran against a 70-year-old man (Sen. S. I. Hayakawa) who looked 80," and lost.

If you or someone in your family has a personal problem such as alcoholism, "that's almost a plus in this age of People magazine." The "macho cool candidate," he says, has been replaced by the one who admits to family troubles.

Greenfield advises the candidate to polish up his or her speaking style. "The great speech," he says, "is as powerful a weapon now as it has ever been."

He considers Ronald Reagan, "whatever your opinion of his politics," as "the most gifted political rhetorician of our time. He has a sense of how words can ignite the attention and the enthusiasm of his audience."

He disagrees with the "myth" that "if you're ahead, don't debate." People now expect you to debate. I like that."

Debating, he adds, "is one of the easiest things. You have to work at it to lose."

No matter how a candidate does on the platform, Greenfield advises making sure -- while the press is watching -- that aides are congratulatory. In no case, should an aide look "as if he has just suffered an attack of kidney stones."

Despite opinions to the contrary, Greenfield believes -- even in this age of the high-powered TV ad -- that "issues decide elections."

"The most important thing I learned," he says, "is that you cannot sell candidates like soap. Soap doesn't vote. Soap doesn't make dumb speeches."