At this early stage, public opinion polls provide flimsy evidence for predicting who the next president will be. One month President Reagan appears ahead of Walter F. Mondale and John Glenn, only to fall behind one or both of the leading Democrats the next month.

Yet the polls are spelling out the probable nature of the 1984 presidential campaign in lush detail. What they are promising--assuming that Reagan seeks reelection--is a very sharply defined campaign, regardless of whom the Democrats nominate.

The polls are telling us, for example, that issues are likely to dominate the 1984 election for the great majority of voters, despite the conventional wisdom that issues are not very important in American elections.

Political partisanship is likely to play a greater role than in any recent election, with far fewer Democrats crossing over to vote for Reagan than in 1980.

There is likely to be bloc voting by economic class beyond any recent experience. People with household incomes of more than $30,000 a year are likely to go Republican more than they usually do, with those with incomes of less than $20,000 more strongly Democratic. The battle will be for the large middle group, those earning between $20,000 and $30,000.

No candidate has been nominated, but almost everyone is pretty sure of how he or she will vote if Reagan runs. People tend to be for Reagan or against him, and it doesn't much matter who the Democrat is.

In the most recent Washington Post-ABC News poll, for example, 95 percent of registered voters interviewed were able to choose between Reagan and Mondale without prompting; 93 percent were able to choose between Reagan and Glenn. At most, that is, only 7 percent said they were undecided 15 months before the election.

No doubt many voters will change their minds, but it appears that the vast majority of voters regard the 1984 presidential choice as one that is simple and clear-cut. The public is divided into one camp of hard-core Reagan supporters, another of hard-core opponents, and a relatively small group of swing voters--not more than a third or so of the electorate--that will decide the outcome.

Thirty to 40 percent of the public may be judged strong Reagan backers, depending on how rigidly the term is defined. A profile shows them to be 99 percent white, mostly male, tending to live in the South or West, coming from the middle or upper-middle class.

The number of hard-core opponents appears to be marginally fewer. Almost all blacks are included among them, a majority are female, they are found most often in the East, they are predominantly working class, and are especially pronounced among those with household incomes of less than $15,000 a year.

But elections are not left to those with intense feelings alone. There remains that other third or so of the electorate who are most susceptible to changing their minds. Only loosely tied to political life, with many not committed to voting at all, they tend to move in a wave, and they have created the small swells and ebbs in Reagan's popularity rating.

In answer to pollsters' questions, these people emerge as "independents" rather than Democrats and Republicans, as "moderates" and not liberals or conservatives. In fact, many are neither independent nor moderate, they simply are not involved one way or the other. They know less about public affairs and tend to be more cynical about politics than the rest.

These citizens are far different from the large pool of unregistered or nonvoting blacks that the black activist, the Rev. Jesse L. Jackson, is attempting to rally. The blacks, beyond doubt, would vote overwhelmingly for a Democrat.

There is no such certainty regarding the other uninvolved. Many of them are young, and while they are not tuned in to politics, they are as likely to vote Republican as Democratic, if they get to the polls at all.

Of course, the outlook may be very different by next summer or even sooner, depending on events. At this stage it is not even clear that Reagan will seek reelection. For now, though, the polls are spelling out a campaign that seems more clear-cut than most.