Suddenly, the issue of Social Security is dominating the presidential campaign. Should Walter Mondale even hint that Ronald Reagan is even thinking of cutting the program, presidential spokesman Larry Speakes leaps to his lectern and promises -- nay vows -- there will be no such thing. Reagan, for whom Social Security long exemplified a government descending into the muck of socialism, has been born again as a New Democrat.

The non-debate on Social Security teaches us some things about this country. Like Dwight Eisenhower before him, Reagan has learned that the New Deal is not to be trifled with. The president is a slow learner, but it's finally dawned on him that what makes the New Deal programs so everlastingly popular is that they were, by and large, directed toward the vast middle class. You cannot, on pain of political death, mess with those benefits.

The other lesson, though, is that you can mess with the benefit programs of the Great Society. Those were by and large directed at the poor. These programs can be reduced, abolished, denigrated and mocked. Indeed, Reagan has managed to cut back on food stamps, welfare, legal services and a host of other programs. All it has earned him is the undying enmity of blacks and other minorities -- and a lead in the polls that would be the envy of any politician.

To be sure, there are some differences between the New Deal programs, particularly Social Security, and most Great Society programs. Social Security is considered an insurance program. You pay into it and you get something out. It is widely viewed as a contractual arrangement -- certainly moral, probably legal. Great Society programs, on the other hand, smack of government largess, welfare. They might be deserved, but they're certainly not earned.

Nevertheless, there are other programs directed toward the middle class that are in no way insurance programs. Take government student loans. No one directly contributes to this program. Yet, of all the programs Reagan has trifled with, this one may have cost him the most politically. That's because the middle class in this country is like the proverbial 8-foot- tall gorilla. Don't take away its toys.

Now Mondale is remembering the lesson all Democrats learn at birth. For a while -- too long, his critics say -- he talked of fairness, listing the Great Society programs Reagan cut. The result was a resounding yawn heard the width and breadth of the nation. As a sleep-inducer, the fairness issue was second only to the deficit. Put the two together and they became a soporific overdose of Rip Van Winkle proportions.

But once Mondale brought up Social Security, and the entire nation raised itself from its stupor. This was not fairness, this was not the deficit, this was something that really mattered -- the continued pampering of the middle class. And Mondale was clever enough not to limit his dire warnings to those already retired or nearly so. He warned about Reagan's alleged long-term intentions -- and the president was quick to extend assurances to virtually, as they say in pro-life circles, the unborn. That's not surprising. The voting habits of the middle-aged and middle-class are not hypothetical. People aged 55 to 74 are nearly twice as likely to vote as those aged 18-29. Buzz-off, Yuppies.

The upshot is that between Social Security ($260 billion), which is inviolable, and the defense budget ($270 billion), which is nearly so, about half the proposed total federal budget for 1985 ($925.4 billion) is beyond debate. Neither candidate will seriously discuss cutting either one -- nor what that means to the poor and the disadvantaged, not to mention the young. Spring has turned to fall and the candidates have turned to Social Security. It is what it is and has always been -- an evergreen.