Tuesday night's debate demonstrated with stunning clarity why it is so hard for so many people to decide whether Ronald Reagan or Jimmy Carter should be president for the next four years. Both showed their skills as campaigners and, equally, both showed the deficiencies that might deter anyone from wishing either of them was president.

Carter, for his part, made a series of targeted appeals to specific constituencies -- answering one question as a traditional Democrat, another as a friend of Israel, a third as the champion of blacks and Hispanics, a fourth as the son of the South.

But at no time did he combine those specific appeals into a broad agenda for America, and that has been the failing of his administration. His former speechwriter, Jim Fallows, observed that Carter has "many policies, but no one policy." As Elizabeth Drew of The New Yorker put it, "He is constantly blurring the picture."

Reagan, on the other hand, paints in broad strokes with his promises to "get government off your back" and "make America respected throughout the world." The trouble is that his picture is more a romanticized notion of the 1920s than a reflection of realities in the 1980s. So it needs constant readjustment, the kind of policy realignment Reagan made almost weekly during the campaign and would be forced to make as frequently as president.

Carter's presidency has confirmed the skepticism of his early crisis.He has botched his relationship with Congress as thoroughly as he burned his bridges with the Georgia legislature. He has confused the bureaucracy (and, often, his own staff) by his introverted decision-making, just as he did in Georgia.

Off his record in California, Reagan is a better bet as a political executive. He is a superior rhetorician and persuader of the public, and he has shown more skill than Carter in searching out able people and delegating authority to them.

But it is doubtful that at age 69 he has the mental energy and drive to put his own stamp on an unfamiliar government. He is as chary of legislative bargaining as Carter, and he is probably not Carter's match in those unique tests of presidential leadership -- the Camp David-type negotiations where the outcome may depend on the skill and stamina, the knowledge and perseverance of the man speaking for the United States.

It is impossible to determine which of these men would be the more capable president, because the evidence is strong in Carter's case and suggestive in Reagan's that neither has the range of skills and instincts the job requires. The best that can be said of them is that they have both selected as running mates and possible successors men who are exceptionally experienced and skilled in the arts of government.

The campaign has focused on economics and national security. Carter's greatest failure is the fact that his own constituents have suffered a decline in their real income during his tenure in office -- 7.4 percent for the typical factory worker with three dependents -- and a real increase in taxes to boot.

But the deep tax cuts that Reagan promises as relief to Carter's victims strike even conservative economists as a dubious cure for persistent inflation. His proposal to dismantle an energy policy that is finally beginning to reduce dependence on foreign oil is counterproductive. Reagan's claim that he would balance the budget by "cutting fat" deserves no more credence than Carter's 1976 promise to streamline government by consolidating agencies.

Reagan's election would likely mean a boost in Pentagon budgets, a change some consider necessary and others extravagant.Carter's claim that it would be dangerous to follow Reagan's suggestion of scrapping the stalled SALT II treaty with Reagan and starting talks afresh would be more credible if he were not taking national security advice himself from Zbigniew Brzezinski, a man who deliberately set out to "shock" the Kremlin in 1977 by scrapping the almost completed Ford-Brezhnev version of SALT II and thereby lost at least three years in the vital battle for arms control.

There is one area of policy, less discussed, where the differences between Reagan and Carter are genuine and significant -- the area of law and social order. A consistent thread of Carter's career is his tendency to see the law as a tool for change and an instrument for seeking social justice. This belief, more than any other, has brought him occasionally to the liberal pole of politics and has made him the agent for elevating to power in the judiciary and regulatory arms of government people with a strong personal commitment to changing the status quo.

Here, the contrast with Reagan is complete. Whether one thinks of his past criticism of civil rights bills and campus anti-war dissent or his present views on ERA, abortion and affirmative action, Reagan has consistently spoken in defense of the status quo, and has brought to power -- through appointments -- judges and regulators who view the law as a bulwark against what they see as disruptive or dangerous social change.

Those who recognize that the appointive power is one that even a president of limited abilities can use with enormous effect may find in this divergence a basis for casting a reluctant vote.