A spoonful of disbelief a day before the election:

Of course it matters which one wins tomorrow -- President Carter or Ronald Reagan -- but our choice is not nearly as stark as either their rhetoric or our system of political labeling may have led us to believe.

Whatever their genuine differences of outlook and temperament, both men long ago began the process of trimming their positions to suit perceived public opinion and to satisfy the interest groups that, along with Congress, are the permanent government.

Carter lays daily stress on his commitment to a strong defense. Reagan the pro-business deregulator goes out of his way to reassure labor that he would not dismantle the derided Occupational Safety and Health Administration. Farmers, old people, maritime unions -- all are carefully reassured by both candidates that they will not lose their subsidies and benefits.

On most points of domestic policy, Carter and Reagan are already more alike than different: Indeed, it could be argued that one problem each has had in this campaign has been differentiating himself from the other.

And there are at least two other reasons for thinking a Carter administration and Reagan administration would not be that different:

Congress is one. Power may change hands at one end of Pennsylvania Avenue this week. It won't at the other. The chairmen, the committee structures, the interplay of interests in the House and Senate will be about the same the next four years as they were the last four, and about as receptive to change. You need only recall 1976 and candidate Jimmy Carter, outsider, who called our tax code a national disgrace and said he would move to have it rewritten top to bottom.

But presidents don't write tax laws by themselves, and it turned out that the Ways and Means and Finance committees that had written ours over the years liked them fine about the way they were. Carter never bothered even to send a full-blown tax reform bill to Capitol Hill.

The second factor is outside influences. The Organization of Petroleum Exporting Countries will do more to determine the health of the U.S. economy over the next four years than whoever is president. The wheat harvest, not the philosophy of the secretary of argiculture, will determine food prices.

Presidents are indeed powerful. They do help shape the future. But they do so only at the margin. Campaign shorthand and campaign symbolism are misleading; they lead us to expect too much. Carter's forgotten tax promises of four years ago are one reminder. The broad domestic record of the Nixon administration may be another.

As Reagan has done in 1980, so Richcard Nixon in 1968 presented himself as an antispending, antiregulation Republican who was going to reduce the federal presence in everyday life in America. There would be a New American Revolution, a flow of power back to the state and local level, Nixon promised.

Revenue sharing was offered up as a symbol of this, and Nixon did get a grudging Congress to enact revenue sharing. He also succeeded in dismantling a contrary symbol, the antipoverty Office of Economic Opportunity he inherited from Lyndon Johnson. And the regulatory impulse was partly restrained in his administration. OSHA, for example, never got off the ground under Nixon.

Yet there is another side to the record. It was the Nixon administration, in a spirit of tidiness, that created the Environmental Protection Agency to centralize enforcement of environmental laws. The first administrators, William Ruckelshaus and Russell Train, are regarded by environmentalists today as having been public-spirited, energetic men who did their job forcefully and well.

As to spending, under Nixon and Gerald Ford domestic spending soared (even while defense spending, as Carter observed in last Tuesday's debate with Reagan, was declining in real terms, after adjustment for inflation). It was in the Nixon years that the Social Security program, the government's largest, was "indexed," as the economists say, so that benefits would rise automatically with inflation. No other single provision of law has done as much in the eight years since to lift government outlays.

It was also in the Nixon years that the mixture of subsidized housing programs for the poor handed down from the Kennedy-Johnson era was laid aside in favor of a new Republican housing program, rent supplements. Much was made of this at the time by the housing secretary, James T. Lynn; it was an example of how the Republicans could rationalize the government. Lynn later went on to become budget officer and is still regarded as a leading expert on economy in government among Republicans. And meanwhile, what is the fastest growing of all federal welfare programs for the poor? It is the Republican rent supplement program that economizer Lynn helped put in place. The subsidized housing program will soon cost more than food stamps, according to Office of Management and Budget projections.

Plainly presidents matter a great deal in foreign policy. They also wield lasting power when they nominate Supreme Court justices, and the next president may have more than the usual number of vacancies to fill on the high court. But in other areas of policy, there is more of a blur. What presidents mainly do is find new bottles from which to decant the same old wine.

They are glassblowers.