Correction: Earlier versions of this review misspelled the name of Mad magazine’s mascot, Alfred E. Neuman. This version has been corrected.

Okay, right up front, I confess: I’m a longtime Jeff Greenfield fan. Tune in any election night and there he is, Alfred E. Neuman plunked down amid a roomful of nattering pundits on CNN or one of the real networks, and as I’ve remarked to my wife more than once, he is the only one I’d be happy to invite to our kitchen table.

So it was with a smile of my own that I learned Greenfield has written a new book devoted to one of my favorite genres, alternate history, or as the academics now call it, counterfactual history. Debating the “what ifs’’ of history has always been catnip for intellectuals and the media alike, and since Greenfield is a member of both clubs, it’s no surprise that “Then Everything Changed’’ is not only thoughtful and sophisticated, but marked by the author’s evident enthusiasm. Greenfield is having fun here, and you can feel it on every page.

“Then Everything Changed’’ is composed of three novella-length counterfactual scenarios, each centered on a modern presidency: a December 1960 assassination of president-elect John F. Kennedy, which forces Lyndon Johnson rather than Kennedy into the Situation Room for the Bay of Pigs and the Cuban Missile Crisis; the failure of Sirhan Sirhan to assassinate Robert F. Kennedy in 1968, which allows Greenfield to imagine a full Kennedy campaign against Eugene McCarthy and then Richard Nixon; and a comeback victory by Gerald Ford against Jimmy Carter in 1976, which posits how Ford might have handled the Iran hostage crisis, among other things.

The weakness of too many counterfactual essays and books is that they tend to be, at best, implausible and, at worst, silly. Simply imagining that a 15th-century Chinese explorer discovered California doesn’t make the Chinese colonization of America logical. Greenfield’s scenarios are relentlessly grounded in plausibility, which is the book’s strength. He may be the first counterfactual essayist to actually go out and interview those involved in his imaginings, the better to explain his characters and their possible actions. A second weakness of counterfactual literature is a general inability to sustain narrative drive — I’m not really sure why this is — which is one reason most scenarios tend to be essay-length. Greenfield isn’t as successful at overcoming this. He writes pretty well (for a TV guy), and his stories are insightful, but they stop short of being page-turners.

That said, this is a book political junkies will adore. The gang’s all here: JFK, LBJ, RFK, Nixon, Richard Daley, Ronald Reagan and all their many, many aides, not to mention cameos by youngsters such as Al Gore, Roger Ailes and Bob Woodward. It’s a mark of Greenfield’s deep knowledge that, other than the RFK campaign, his scenarios aren’t the obvious ones: I had assumed the JFK essay would imagine the president surviving Dealey Plaza, not that he would die earlier, much less at the hands of a would-be assassin. Greenfield rescues from obscurity a crackpot who really did approach Kennedy’s Palm Beach house in 1960 with dynamite strapped to his body. In real life the fellow backed off when he saw the Kennedy family and was arrested. But here: ka-boom.

While the RFK scenario struck me as the most sophisticated, if a tad far-fetched, the early LBJ story is probably the sharpest. I won’t give too much away, but Greenfield argues persuasively that Johnson’s unfamiliarity with foreign affairs and his tendency to trust his generals would have made him more likely to cave in to the likes of a wild-eyed Curtis LeMay during the missile crisis. (Guess that amounts to a spoiler. Yeah, things get nuked.) The LBJ story is marred by one of the few missteps Greenfield makes. At the moment of truth, smack in the middle of squabbling with Khrushchev, he has Johnson exit stage right, leaving it to Vice President Hubert Humphrey to deal with the Russians. This is a bit like Mario Puzo sending Michael Corleone on a Florida vacation the moment Mo Green lies down on his massage table.

About the worst you can say about “Then Everything Changed’’ is that it doesn’t furnish many “wow’’ moments. What you get instead are dozens of “hmmm’’ moments, as well as a keen appreciation of how fragile the course of history truly seems. In fact, I’d look forward to more. What if President Nixon had a happy childhood? What if President Carter hadn’t fought off that attack bunny? What would have happened if Monica Lewinsky went to the dry cleaners more often?

Sequel!

Bryan Burrough is a special correspondent at Vanity Fair and the author of “Public Enemies’’ and “The Big Rich.’’

THEN EVERYTHING CHANGED

Stunning Alternate Histories of American Politics: JFK, RFK, Carter, Ford, Reagan

By Jeff Greenfield

Putnam. 434 pp. $26.95