Until its recent world premiere at Houston's new Wortham Center opera house, almost everyone who had heard about ''Nixon in China'' reacted in much the same way.

It was Sellars, the champion enfant terrible among today's theatrical directors, who first came up with the idea of basing an opera on former President Richard M. Nixon's visit to the People's Republic of China in February 1972.

That was five years ago, a mere decade after the historic event had taken place. Chou En-lai and Mao Tse-tung had both died in 1976. Nixon and his wife, Pat, however, were still very much alive. So was Henry Kissinger, who as secretary of state laid the groundwork for the Nixon visit during a secret trip to China in 1971.

Although the annals of opera contain little precedent for it, the idea of turning still-living persons into larger-than-life operatic characters was not what struck Adams as preposterous. What put him off was Sellars' vision of Nixon, Mao and the others as ''the mythological figures of our time.''

Perhaps the merciless Nixon caricatures on ''Saturday Night Live'' had made too much of an impression. Perhaps the Watergate affair was too fresh in Adams' mind for him to take seriously the notion that Nixon could be portrayed in any way sympathetically, as a romantic hero whose public ambition puts him in painful conflict with his non-public sensibility. In any case, Adams stalled. It was only late in 1984 that he - and librettist Alice Goodman - agreed even to consider the possibilities.

The possibilities, it turns out, are great. And that's why ''Nixon in China,'' instead of being just another easily dismissed operatic flop, is a failure of the sort that brings tears of regret to a critic's eyes even as he's wielding the hatchet.

However unviable it may seem at first glance, Sellars' germinal idea is in fact quite brilliant.

As conceived by Sellars, ''Nixon in China'' is anything but heavy satire. The inspiration came largely from Kissinger's memoirs, a document that points out again and again that the private Nixon indeed was - and doubtless still is - a sensitive, vulnerable man.

According to Kissinger's account, Nixon regarded the China visit not just as an important diplomatic mission and a political coup but also as a deeply meaningful personal gesture. For Nixon, the extension of a hand to Chou En-lai on the runway of the Peking airport on that cold winter morning in 1972 represented all he had learned about peace and brotherhood during his Quaker upbringing.

He felt certain of what he was doing yet uncertain about the results, Kissinger informs us. The press reported only on the smiles that decorated the summit meeting's grandiose public events; the feelings the former president shared with his intimates, Kissinger says, ran the gamut from elation to despair.

This is such stuff as operas are made on.

And Alice Goodman has shaped it into a libretto that is as literarily beautiful as it is structurally sound. The rhymed and metered text is ''fastidiously accurate'' as to what was actually said during the China visit, Goodman claims.

At the same time, its mixture of cryptic Chinese speeches and down-homey American conversations adds up to enormously resonant poetry. Goodman's contribution to ''Nixon in China'' works superbly as an operatic script; in the bargain, it rewards thoughtful reading and re-reading in silence.

''Nixon in China,'' alas, does not transpire in silence. It's a full-length opera, and for its entire 2 1/2 hours of running time, its considerable theatrical potential is countered by the music of John Adams.

Adams is a San Francisco composer who rose to fame in the late 1970s as a second-generation Minimalist but who lately has taken a turn toward neo-Romanticism. One suspects Sellars picked him for the project on the basis of ''Harmonium,'' a big choral/orchestral work from 1981 that sets poetry by Emily Dickinson and John Donne in a way that is indeed dramatic.

Most Adams watchers figured the composer was jocularly celebrating the end of his involvement with Minimalism when in 1982 he unveiled ''Grand Pianola Music,'' a loud, garish and deliberately banal piece of pop-colored fluff that carries the idea of minimal rhythms and harmonies to an absurd conclusion.

''Light Over Water,'' a 1983 ballet score written for the Lucinda Childs dance company, demonstrated that Adams was not ready to abandon completely what he called Minimalism's characteristic ''great prairies of non-event.'' But Adams' 1985 ''Harmonielehre,'' an orchestral rhapsody of Straussian dimension and impact, convinced listeners that Adams really had effected a rapprochement with traditional musical devices.

''Harmonielehre'' contains graspable melodies; more significant, it contains harmonies that move purposefully toward points of climax and resolution. Like almost all music composed since 1700, but unlike the vast majority of Minimalist pieces, ''Harmonielehre'' deals with music in a quasi-rhetorical way.

Instead of demanding that it simply be experienced on its own terms, ''Harmonielehre'' makes a statement in terms that have long been a part of Western culture. Its contents remain to a certain extent modernistic, but the manner in which the contents are treated is actually quite conventional.

To judge from its libretto as well as from the straightforward staging Sellars has designed for it, ''Nixon in China'' is a work whose music demands a similarly conventional treatment. Adams seemed right for the job.

But instead of following through on the promise of ''Harmonielehre,'' Adams retrenched. With all its changes of tempo and texture, the score for ''Nixon in China'' only appears to hold to conventional operatic format; in fact, most of its segments are very much in the Minimalist vein, and most of them traffic heavily in Adams' worn-out Minimalist clichés.

Next to the uncanny physical resemblance baritone James Maddalena bore to the title character he was portraying, the first thing that struck me during the Oct. 22 premiere of ''Nixon in China'' was how weak the music sounded in comparison to the theatrical music of Philip Glass.

Glass, several of whose operas were discussed on this page four weeks ago, is a composer who knows how to shape relatively static music into large structures that, in the long run, are powerfully dynamic. In ''Nixon in China,'' dynamism is found only on the small scale, and it's the opera as a whole that doesn't go anywhere.

The scenes are differentiated, to be sure. Each of the main singers (Maddalena as Nixon, soprano Carolann Page as Pat Nixon, bass-baritone Thomas Hammons as Kissinger, tenor John Duykers as Mao, soprano Trudy Ellen Craney as Mao's wife, Chiang Ching, and baritone Sanford Sylvan as Chou) gets a chance to shine. There are several big chorus numbers and a ballet, a takeoff by choreographer Mark Morris on Chiang's infamous ''The Red Detachment of Women.'' The final act contains both a storm sequence and a Hollywood-flavored reverie that - for a few moments, at least - are almost ear-catching.

Occasionally the synthesizer or the brass section makes a sonic splash; most of the time, though, the surface of the music stays inappropriately placid. Goodman's libretto provides the composer with plenty of opportunities for character development. Instead of capitalizing on them, Adams far more often than not sets moods and then merely sustains them. Sellars regards these characters as ''mythic,'' but Goodman has assigned them words and cast them into confrontative situations that make them seem like flesh-and-blood human beings; Adams' music makes them seem like cardboard dolls. The score is cliche-ridden in the abstract; it is even more so when regarded in its theatrical context.

And that's unfortunate.

''Nixon in China,'' which will travel to New York, Washington, D.C., and Amsterdam after its Houston run, is a major effort - well-staged and excellently performed - on the part of an American opera company that's proudly celebrating the opening of its new home. Sellars, who says he's not interested in directorial concepts but only in characters and events, obviously put a great deal of thought into its construction. Goodman, who seems to have understood clearly the point that Sellars was trying to make about how world leaders are, after all, only human, has crafted one of the finest English-language opera librettos the last quarter-century has seen. ''Nixon in China'' has a lot going for it - almost everything, except the music.

Probably no one was surprised when, a few years later, the so-called Achille Lauro affair was done up for television (one version starred Karl Malden as the victim, another featured Burt Lancaster). It seemed unthinkable, though, that the subject could be translated into opera.

John Adams' "The Death of Klinghoffer" deals only marginally with Klinghoffer's death. This is an opera about idealism and the conflict that is idealism's inevitable byproduct. While the idealism and conflict depicted here are indeed specific, they are distilled to the point of abstraction. The antagonists' actual goal seems never so important as the fact that they have a goal; the actual obstacle with which they collide seems almost insignificant compared with the immovability of obstacles in general. Instead of the gory thriller a lesser crew might have put together, Adams and his colleagues - librettist Alice Goodman, stage director Peter Sellars, choreographer Mark Morris - have concocted an operatic parable for modern times. Its moral, as suits today's confusion, is open-ended. There is no excusing terrorism, but often it happens that a sense of righteousness exists on both sides of a disputed fence.

"The Death of Klinghoffer" was accused of pro-Israeli sentiment when it was premiered in Brussels in the spring of last year, and protests from the Arab community continued seven months later during a run at the Brooklyn Academy of Music. But the opera was also charged with being pro-Palestinian, even anti-Semitic. The loudest objections of the latter sort were aimed at a scene in the prologue that showed the Klinghoffers visiting with another Jewish couple at their home in New Jersey. For reasons that have never been publicly explained, Adams and company decided to drop that scene. It was included in the Brooklyn production but omitted from the recording just released on the Nonesuch label. The production at the San Francisco Opera last month marked the first time "The Death of Klinghoffer" was presented on stage in its official revised version. Only those who witnessed it can say if the domestic scene was or was not offensive.

The San Francisco production, in any case, seemed perfectly even-handed. As it stands now, the prologue begins with the poignant "Chorus of Exiled Palestinians" and then moves straight into the equally poignant "Chorus of Exiled Jews." After that come two acts of soliloquies and dialogues fitted loosely around the details of the hijacking and the murder. In terms of the shipboard action, only the most biased of operagoers would disagree as to who the good guys and bad guys are. At the same time, only the most callous could sit through the performance and not be reminded that most problems are far more complex than they appear.

The team that created "The Death of Klinghoffer" was also responsible, five years ago, for "Nixon in China." Like the Achille Lauro affair, former President Richard Nixon's visit with Mao Tse-tung in Beijing in 1972 at first struck the opera world as something too contemporary - and too political - for theatrical treatment. "Nixon," however, proved an immediate success. Adams' music, a form of modified minimalism softened with superbly lyric melody, had a lot to do with it. But so did the plot, which was conceived by Sellars and fleshed out by Goodman. To the surprise of many, the writers had no ax to grind. Nixon was presented in a romantic, almost heroic, light; he was portrayed not as a flawed president but as a hopeful human being, someone worried about failing yet still willing to extend the hand of friendship.

"Klinghoffer" is not so simple. The story line is just a thread in a fabric whose message, as described above, has little to do with the murder of its title character. Goodman's libretto - layered with metaphor, filled with allusion to both the Bible and the Koran - makes her previous effort seem almost prosaic. The chorus plays a major role here, and its members wear neither street clothes nor makeup; in the prologue they clearly represent Palestinians and then Jews, but most of the time they voice the frustrations of Everyman. Morris' choreography, an incidental feature of "Nixon in China," is constantly on view in "Klinghoffer"; often the main characters are limned simultaneously by a singer and a dancer, and sometimes there is telling dissonance between the performers' statements.

Whereas the sets for "Nixon" were in essence realistic, the single piece for "Klinghoffer" - a huge scaffold designed by George Tsypin - serves as a metallic skeleton for ships, nations, cultures and whatever else the text brings to mind.

Adams' score, too, is rich in ambiguity. Individual lines are meaningful vis-a-vis the plot, but they take on deeper meaning if the plot can be pushed into the background. Enemies are distinguished by their actions, not by their modes of expression; at least in the music, interchange between terrorists and victims is more philosophical than confrontative, as much compassionate as passionate. Melody, in the traditional sense, almost never enters the "Klinghoffer" picture. Adams' prime concern here is with harmony, tensed and relaxed - but mostly tensed - over the opera's entire duration. Except for choruses and the angry aria sung by the character of Marilyn Klinghoffer at the opera's end, nothing in the score seems viable as an excerpt. The elements make sense only when heard as part of a long, long flow.