OUR AILING PARTY system, which seems to be dying of the post-1968 "reforms" that were supposed to give it new life, has needed a master diagnostician for some time. In Nelson Polsby it has at last found one.

This account of how party reform changed the political landscape is political science as it ought to be practiced but seldom is--the most searching work of analysis of its sort that I have read in years. It might be extravagant, but only barely so, to call Nelson Polsby the Newton of the post-1968 political dynamics. All who would play the game of party revival (sometimes known as "putting the toothpaste back in the tube") will need to digest Consequences of Party Reform..

It isn't that Polsby thinks the political world was suddenly turned upside down in 1968. A trend towards the "automated, nondeliberative" nominating convention had actually set in years earlier. The Democratic Convention of 1952 was the last major party conclave (of 15) that went beyond a single ballot when it chose Adlai Stevenson.

And, curiously, in the same year the "media candidacies" now encouraged by the new delegate-selection rules had their early precursor in the Estes Kefauver campaign. His televized crime investigations, Polsby suggests, provided him with the first constituency which was fully independent of the accustomed brokers of presidential nominations. Not that it was sufficient to get him nominated that year; primaries did prove to be "eyewash," and before 1968--Polsby reminds us--were generally thought of as a hazardous pursuit for would-be nominees.

Polsby sets the stage for the great eruption of reformism in 1968 by recalling that that year was exceptional in all sorts of ways. Not only was there Vietnam; it was a year of "major civil disturbances" then and now unprecedented.

But how was it that the presidential nomination fight of that year aroused such dissatisfaction, pushing the Democrats into the clutches of the reformers? One major factor, Polsby suggests, was the assassination of Robert Kennedy, just as his insurgent candidacy was hitting its stride. Hubert Humphrey was then and thereafter the choice of at least a plurality of Democrats polled, and probably would have won. But Kennedy's sudden removal ended a plausible challenge to him, and deprived the Democrats in Chicago of the usual rituals of reconciliation.

Along with the mob scenes and what came to be thought of as Mayor Daley's rough-stuff, all this inspired the fixed idea--or illusion--that Humphrey had been imposed on an unwilling party. On the strength of that angry perception, the reform fever ran its course. And a curious, often perverse course it was.

Believing that conventions were overbossed and unrepresentative, the Democrats' various reform commissions soon imposed rigid, centralized rules of delegate selection. Their writ was at least questionable; but in the end it didn't matter. Many local party regulars found it less troublesome to submit than to tangle with the credentials bureaucracy.

Richard Nixon, Polsby notes, also played a certain role. Had a Democrat won, perhaps the new "uniform rules of behavior for state delegations everywhere" might have been appealed to him and relaxed. There was no such presidential court of last resort.

And then there was Watergate. While it was not essentially a money scandal--I am still here summarizing Polsby's shrewd and interesting argument--it fanned the passion for public funding of presidential campaigns. Public financing, soon written into law, further weakened grass-roots party organization. As one Gerald Ford campaign manager observed, "it became easier to discourage grass-roots activity than to try to control and report it."

Thus the "presidential" parties began to disintegrate and "entrepreneurial candidacies" (reminiscent, Polsby ingeniously suggests, of Broadway shows) flourished. With the new delegate selection systems, party organization was less and less needed--as were the traditional "mediating structures." The latter yielded power to the new ones, especially the mass media. In theory, the new nominating system was more responsive to popular will; in operation, it was in many ways more elitist than ever before.

In any event, reform rules drastically altered the physics of the nominating process, first within the Democratic Party but later, as state laws were adjusted to new party rules, among Republicans as well.

The effect on government was soon evident. In a fresh analysis of Jimmy Carter's troubles in office, Polsby suggests--nay, demonstrates--that they were to a great extent the result of his "overlearning the lessons of his nomination." Under the new rules, you could get a nomination by courting a bewildering number of factions and interests. But governing remained, in the end, an art of coalition building, especially in Congress. Having played the Lone Ranger, Carter found many of his enterprises friendless.

This, as most serious studies have shown, is the predictable result of a nominating process that vests decisive power in preferential primaries. Primary voters are different; they do not vote as voters vote on election day. And the extremist posturings that primaries encourage become handicaps once the electioneering is over and government must begin. Obvious as this may be, it remains difficult to wean reformists from their illusion that plebiscitory nominating procedures are more "democratic" and also more likely to produce "good government."

And what to do about all this? Diagnosis, not prescription, is the business of Consequences of Party Reform. Indeed, as an institutional Tory, Polsby is rather less optimistic than others that the clock can be turned back a bit. His analysis of some of the nostrums of "re- reform" (for instance, the bunching of primaries in a so- called "window") is as merciless--and gloomy--as his analysis of the follies of reform.

Is it mere nostalgia, then, to pine as many of us do for a more reasonable mix of influences (especially re-emphasizing professional political judgment) in the nominating process? Here, Polsby is more hopeful. A process that restores some measure of peer review might at least be educational. It might not lead to different candidates, but "what (it) can be assured of doing . . . is placing within each candidate's grasp a more enlightened understanding of what it takes to govern."

In any case, I share Polsby's apparent bias (which is not completely concealed by scholarly self-discipline) for some measured return to the "old politics"--strong, decentralized parties with vigorous local roots, in which even elected officials enjoy some influence in the choice of nominees.

The simple fact is that the parties, in the name of reform, tossed too much of value aside after 1968 and desperately need to retrieve it. The retrieval effort, on the evidence assembled here, will be tricky and may not succeed. But no one, I think, will emerge from this excellent investigation doubting the urgent need to give it a try.