The presidential race has taken an unexpected turn -- one that was not foreseen by the strategists on either side. Going into Sunday's final debate in Kansas City, the fulcrum of the election seems to be President Reagan's capacity to make a case for himself on television.

That is a truly astonishing situation. There was no historical precedent for a popular incumbent making himself the issue in the election -- until Reagan bobbled his first debate with Mondale in Louisville. There has not been a presidential election in the modern era that has turned on the question of the president's capacity to do his job as well in the future as he did in the past, not even when an ailing President Roosevelt sought a fourth term six months before his death.

Reagan's chronological age would never have been an issue; he is plainly healthier and fitter than most of us who are his juniors. But voters -- especially those who are themselves approaching Reagan's age or who have parents in that age bracket -- are sensitive to signs of faltering energies and faculties. And that is what some people saw in Reagan a week ago Sunday night.

Democrats who are rejoicing at this turn of events are celebrating prematurely. As this issue is now defined, all Reagan has to do is make a plausible case for his own policies to dissipate the Louisville doubts. He does not have to prove that he has a better grasp on nuclear policy or Middle East politics than Mondale does; that would be a tough, tough challenge.

About all Reagan has to do -- as the issue is now defined -- is express his thoughts in complete sentences, keep Nicaragua and El Salvador straight in his mind, and remember the names of the people he meets at the annual economic summits.

In this respect, his situation is quite similar to that he faced as a challenger in the 1980 debate with Jimmy Carter. The Democratic strategy of desperation that year -- since they acknowledged they could not convince most voters that Carter had been a successful president -- was to depict Reagan as a frightening nuclear-bomber menace.

As Carter strained to do that in the debate, Reagan stayed calm and good- natured. He won the debate in political terms, even though Carter scored most of the debating points.

The Democrats are in almost as desperate a plight in 1984, because they cannot convince most voters that Reagan has been an unsuccessful president. So they want to depict him as a doddering incompetent. They may succeed, but not if Reagan shows up in Kansas City with his tongue and brain connected and his good humor intact.

Meantime, those of us who thought that this might be an election on the issues are apparently doomed to disappointment.

For the first five weeks after Labor Day, the issues were overshadowed by the impact of Reagan's personality. For the last two weeks, the issues have been lost in the debate about Reagan's mental capacity.

Maybe if that question is settled in Kansas City, we can get a fortnight of the kind of campaign we were hoping to cover, but I would not count on it.

Most elections in which the incumbent is running turn on a retrospective judgment of the success or failure of his prior service. That is how Roosevelt won reelection three times, how Eisenhower won in 1956, how Ford lost in 1976 and Carter lost in 1980. The voters looked on their works and pronounced them good or bad.

Some incumbents are lucky enough to have challengers who cannot get past the threshold of credibility as potential presidents. Barry Goldwater in 1964 and George McGovern in 1972 made it easy for Lyndon Johnson and Richard Nixon to gain another four years.

But on reflection, the only elections that turn on a serious debate on future policy directions are those in which two non-incumbents, both judged to be qualified, face each other.

We had that kind of election in 1960, and John Kennedy overcame Nixon by offering a more appealing promise of policies aimed at spurring economic growth and military-diplomatic power.

We had that kind of election again in 1968, in a darker political climate, but Nixon won by promising with more plausibility to end the Vietnam War and curb its incident inflation.

When Mondale began this campaign by saying he wanted to debate the future direction of policy, he was in effect conceding that he was a long-shot. Had he been able to make the case convincingly to most voters that Reagan and his policies had failed, he would have done so. Instead, he chose the harder course of arguing that those policies, however pleasant they might be for most at the moment, would fail in the future.

It is not surprising that proposition has been hard to sell. And it bears underlining that the hope of exploiting a personal weakness in Reagan is the same strategy that failed the Democrats in 1980.

In effect, they are leaving the election in Reagan's hands. I would not bet the rent that he will fumble it.