Behind the presidential mask. Can we say that Ronald Reagan, who still enjoys an almost inexplicable popularity with the American public, really was `unmade' by events of the past four years?

LANDSLIDE: THE UNMAKING OF THE PRESIDENT, 1984-1988 by Jane Mayer & Doyle McManus

Houghton Mifflin. 468 pages. $21.95

`LANDSLIDE'' starts with a corker. It is March 1987, and the Reagan presidency is under the shadow of its darkest hour. The Iran-contra publicity typhoon is washing up against the doors of the Oval Office. Donald Regan has stormed from his job as chief of staff.

And Jim Cannon has just spent the weekend conferring with the battle-fatigued troops in the White House. Cannon is a trusty Republican insider dispatched by Howard Baker - about to assume the chief's mantle - to test the waters at 1600 Pennsylvania Avenue. He can hardly believe what he finds.

``Chaos,'' Cannon later recalls to the authors of ``Landslide.'' ``There was no order in the place. The staff system had just broken down. It had just evaporated.'' More chilling is what White House aides tell of the President himself. ``They told stories of how inattentive and inept the President was. He was lazy; he wasn't interested in the job. They said he wouldn't read the position papers they gave him - even short position papers or documents. They said he wouldn't come over to work - all he wanted to do was watch movies and television at the residence.''

In a state of half disbelief, Cannon types out a memo to Baker suggesting that the new chief of staff ought to consider first whether the 25th Amendment should be invoked to remove an incompetent president from office. If the President is as incompetent as his aides indicate, Cannon argues, his involuntary retirement is the only way to serve the national interest.

Ronald Reagan still is President, of course, and the nation's interests remain largely intact. The following Monday - Baker's first as chief of staff - Cannon and Baker watched Reagan at close range for a few hours, his characteristically chipper demeanor apparently dispelling any doubts about his fitness to govern. ``The old fella just looks dandy,'' Cannon observes wonderingly.

And that's that. During the next 391 pages, ``Landslide'' makes no further mention of that story, nor does it attempt to relate it to the narrative that follows.

What is supposed to follow is the tale of an administration that, through arrogance, incompetence, and indirection, managed to squander its whopping electoral mandate from the 1984 presidential elections. What actually follows is, for the most part, a rehash - albeit a vivid one - of the Iran-contra affair.

The subtitle - ``The Unmaking of the President 1984-1988'' - tries to tell it all. But the text - and, this reviewer would argue, reality - does not sustain the conclusion.

Clearly, ``Landslide'' has much to recommend it. The authors - Jane Mayer, a White House correspondent for the Wall Street Journal, and Doyle McManus, a Washington-based writer for the Los Angeles Times - are first-rate reporters, skilled at providing the kind of insider anecdotes and telling details that make an account such as this so entertaining.

Mayer and McManus, for example, present one of the most telling glimpses of modern-day campaign tactics to be found in any book, as solid a bit of evidence as one is likely to find that the vapidity of modern-day campaigning is entirely premeditated. It comes in the form of an account of a 1984 meeting by top Reagan campaign officials where it was decided that the reelection effort would be best served if issues and specifics were avoided at all costs.

But the book's heart lies with the Iran-contra affair. The authors offer a wonderful telling of the internecine rivalries among the President's men that left a power vacuum at the top and, ultimately, encouraged a handful of lieutenants to precipitate a debacle. In ``Landslide,'' the Reagan White House is a lot like the Romanov court - all whispering and intrigue. By Mayer and McManus's reckoning, the Reagan era was ended by the Iran-contra affair just as surely as the Russian czar was brought down by World War I.

Which is precisely the problem with ``Landslide.'' Of course, we know that the Iran-contra affair was the worst political scandal of the decade. And yet, can we say that Ronald Reagan, who still enjoys an almost inexplicable popularity with the American public, was really ``unmade'' by events of the past four years? Can we compellingly argue that an administration that oversaw the overhaul of the US tax code, negotiated history's first nuclear arms reduction agreement, and engaged in muscular annual budget confrontations with the Congress (all during the second term) had really been swept into the dustbin of political irrelevance?

To this, the authors say ``sort of.'' Reagan, we're told, ``showed little dynamism inside his own White House.'' He didn't like to work hard, delegated too much responsibility to subordinates, and was ``strangely susceptible to the wiles of both his staff and his wife.'' Consequently, ``Reagan's power waned not just at home but also abroad.''

Is there anything new here?

Little except the revelation that the authors skate onto considerably thin ice when they skid from straight reporting over to analysis. No doubt, Reagan has been a disengaged chief executive. By any conventional standard, perhaps, such disengagement might doom a presidency. And it is this conventional standard to which Mayer and McManus cling.

``The Iran-contra affair revealed [Reagan's] rhetoric to be disconnected from his actions, and his actions to be disconnected from his policies,'' they observe. ``His unmasking was his unmaking.''

But as Reagan has so often defied his critics - and his critics' conventional evaluations of performance - so has he confounded Mayer and McManus. Reagan may have been unmasked. But unmade?

Come on. Ask George Bush, the President's loyal understudy, who may become the first sitting vice-president to assume the presidency since Martin Van Buren. In no small part, Bush's success springs from Reagan's enduring popularity.