IN ONE VERY IMPORTANT sense, former California governor Ronald Reagan has already won the campaign that he formally enters tonight in New York.

For in fact, Mr. Reagan, more than any other national figure, has determined the public agenda for the presidential campaign of 1980. Since 1964, he has been arguing -- articulately and emphatically -- the need to cut the size, scope and spending of government. Perennial critic of government waste and "social tinkering and experimentation," Mr. Reagan now finds his theme song being sung (with only slight variations) by every Republican challenger for the nomination with the exception of Rep. John Anderson. As the advocate of a more muscular national defense and a more skeptical approach to the Soviets, Mr. Reagan has now been joined by almost all of his own party and many of the Democrats.

In a way, Mr. Reagan's position is comparable to that of Adlai E. Stevenson 20 years ago. Mr. Stevenson's views that year -- in particular, on the nuclear test ban treaty and the completetion of the New Deal -- were adopted by his fellow Democrats and became the Litany for two Democratic presidents. Like former governor Stevenson, former governor Reagan's candidacy has excited and enlisted legions of political amateurs whose cause is different but whose commitment is idential.

Now comes Mr. Reagan's announcement speech. He has chosen the battlefield and determined the debate. Where do we go from here and how would a President Reagan take us there? The questions that persist about Ronald Reagan are: What is the larger vision of the nation and the future that he offers beyond his vivid criticism of the welfare queen in designer jeans? Is the Reagan answer to double-digit inflation a national Proposition 13 -- and what would his prescription be for dealing with, say, the trouble in Iran?

One real advantage of the lengthy presidential campaigns is that we can see the candidates' reactions to real questions and real crises. We can examine the candidates' instincts and assess their judgments made -- as a president must make them -- under pressure and never with completely satisfatory information.

The 1976 Regan campaign chose to emphasize the California's opposition to the return of the Panama Canal and, in the process, elevated that transaction to the national center stage and made opposition to the treaty a litmus test for Republican presidental candidates in 1980.

Gov. Reagan's announcement speech and subsequent appearances will probably tell us more about the shape of the 1980 campaign and its emphasis than will those of any other candidate. The very fact that the principal criticism of his candidacy has centered on his age rather than his ideology is strong proof of how the philosophy of Ronald Reagan has become the orthodoxy of the Grand Old Party in 1979.