JOBS, for a growing population, will necessarily be the basic ingredient of social progress over the next four years. And equality of opportunity is most easily approached in an expanding economy. But when the improvement of one person's chances requires pushing someone else aside, the quarrels become politically insoluble. For young people entering the job market, for blacks and Hispanics, for women for just about everybody, movement toward the ideal of equal access requires a rising number of jobs. But how fast can the economy grow? That's a pressing question for the administration.

The American economy has generated new jobs faster in the Carter aministration than in any of its predecessors for the past generation. But President Carter has used methods that brought with them a level of inflation that now threatens the stability of the whole system. That's the dilemma that either he or Mr. Reagan will have to confront.

On Inauguration Day next January, the unemployment rate will probably bw somewhere around last month's 7.5 percent. Inflation will be running close to 10 percent a year. Most Americans consider both numbers intolerably high. What's the president to do?

Mr. Reagan wants to begin with a huge tax cut, most of it in individual income tax rates. Congress is not likely to give it to him. It refused Mr. Carter even a modest $50 tax rebate in early 1977, and since then Congress has become more uneasy than ever about inflationary budget deficits. In response to questions about inflation, Mr. Reagan says that he would reduce federal spending -- but he has provided no details. Mr. Carter promised the same thing four years ago, incidentally, but hasn't managed to do it.

Mr. Carter is also calling for a tax cut. But he wants to keep it smaller than Mr. Reagan's and designed mainly to give business more incentive to invest. The payoff, in higher production and productivity, would come only slowly. Regarding inflation, menawhile, Mr. Carter does not seem currently to have any new ideas. John Anderson wants to try using tax incentives to hold down wage increases. Mr. Carter has hinted at something of that sort -- but, again, Congress doesn't seem to care much for the concept. When Mr. Carter first proposed it two years ago, the legislation sank rapidly out of sight.

Comparing campaign statements is not necessarily the best way to arrive at a choice among these candidates. In purely technical terms, Mr. Carter's position is the most realistic. But it is also the narrowest, and suggests very little change from present methods and policies. Both Mr. Reagan and Mr. Anderson argue that they stand for real change. But the more interesting question is whether they would have, in practice, the stamina to carry out those pledges in the face of the fierce pressures with which Mr. Carter has repeatedly been forced to compromise.

Present campaign positions make less difference than future ability to work with Congress and to convey to the country a clear and steady purpose. On both counts, Mr. Carter's score over the past three and a half years has been low. Against that, it's necessary to weigh the bottomless uncertainties about Mr. Reagan's next moves when Congress refuses to enact his sweeping tax and budget cuts.

In economic policy, the president's primary job over these next four years will be to explain to Americans what's necessary and why. One great lesson of the 1970s for economic policy is that a president's program does not succeed when people neither trust nor understand the man who proposes it. To put it the other way around, economic policy works best when people think that it's likely to work. The performance on the economy in the 1980s will depend less on technical details than on the president's ability to convey a generous and steady sense of common national purpose.