A POLITICAL convention is not merely a stage for a party's actors, a celebration for its faithful or a launching pad for its candidates and programs. It is also a partly calculated, partly flying-blind gamble on the nature and the temper of the country. This requires a relatively small group of Americans to come together briefly, to submit themselves to a process of deliberation whose chemistry cannot be anticipated, and to take a measure of what is on the minds at that moment of nearly a quarter of a billion people. Shortly thereafter, their judgment is submitted to remorseless public scrutiny. It is, or course, precisely this sense that much more than a candidate is being chosen -- that an occasion of rare self-revelation is at hand -- that accounts for the special fascination for conventions, even when, as in the case of the republican convention opening tomorrow, the candidate is already known.

In this underlying sense, Detroit holds no small amount of suspense. For Ronald Reagan's Republican Party -- which is what the primary marathon and Mr. Reagan's own ministrations and the platform debate have now unquestionably made it -- is taking a particularly audacious chance on the sort of society the United States is today. The GOP is figuring that, while it as a party has remained generally true to the traditions that have kept it a minority party for most of this century, the country has changed: the right wing is looking like the mainstream.

The party is betting that the country has suffered enough disenchantment with the fruits and the process of the welfare state to look with a new receptivity upon Republican theories of the liberation of the economy from official overregulation and restraint. It is betting that the country is disturbed enough by the disruptions of contemporary enough by the disruptions of contemporary life to give the GOP a near monopoly on what has been called "the social issue," the anxieties stirred in many formerly comfortable citizens by momentous (and sometimes bizarre or terrible) social change. It is betting that changes in the international balance of power and in the security prospects of the United States have validated the priority that the Republican Party has long accorded to a strong defense and a muscular foreign policy. The party is betting, in short, that the country is now a truly conservative place -- rendered ready by disillusionment and adversity to return to ways associated with better and surer days.

Some Democrats, reading the new Republican platform and observing the stress the Reagan camp is placing on "loyalty" and orthodoxy, have already allowed themselves to conclude that the GOP's familiar "death wish" is again in evidence: better right-wing than president. But the judgment seems to us premature -- and also highly wishful on the part of those who have reached it. The platform tells you where, ideologically, the Republicans are, but it does not tell you where the American people as a whole are. In some of its more publicized and controversial aspects, the platform suggests a narrow orientation. But on repeated occasions the candidate has shown a feeling for a considerable broader part of the political spectrum. He may be a man of the right, but it was not on that basis that he swept most of the primaries, and it will be a surprise if he chooses to play an ideological type in the convention and then in the campaign. In his choice of a running mate, he also has the opportunity to move away from ideology as a principle coloring his campaign, and to inject other tests, such as competence and national experience and, on another level, regional diversity and relative youth.

So on to Detroit, where the Republicans will now reveal more precisely just what they think the questions are to which Ronald Reagan is the answer.