As presidential candidates enter the ring to debate Thursday, the most important question will not be one asked by the moderator. Viewers will ask themselves which candidate gives them most assurance he can solve the problems of Iraq and the all-inclusive war on terrorism.

Looking back at 40 years of presidential debates, history indicates the most important factor will be the viewer's judgment of the strength of leadership the candidates display. Many issues will be discussed, but, as in so many years past, winning and losing will hinge on voter evaluation of the leadership qualities of the candidate who will be sworn in next January as commander in chief.

Politicians have learned much since John F. Kennedy and Richard M. Nixon held their first debate in 1960, but the basic lessons from that first encounter still apply.

In 1960 it was thought that the largest TV audience would be generated for the fourth and final debate over foreign policy, but more people watched the first debate on domestic policy than any that followed. That will be the case again this year, but the first issue will be foreign policy. In 1960, had the fourth debate drawn a larger audience than the first, Nixon probably would have won the election. Those who heard the first debate on radio thought Nixon had won, but those who viewed it on TV believed Kennedy had emerged on top. The lesson was that on television, appearance and mannerisms count heavily, outweighing the factor of logic, which determines the outcome of most academic debates.

It is the first debate that usually counts the most, and this year, when there is far more than the usual animosity toward each candidate, personal impressions of the strength of the two men will count more than the content of the debates. The importance the candidates place on this week's debate was illustrated when each contender started on Sunday taking time from the campaign to begin preparations. At the time of the Nixon-Kennedy debate, Kennedy devoted only one day and Nixon a half day to concentrated preparation.

As we approached the 1960 debates, none of us on the campaign staffs understood what factors would influence the voters most. Immediately after the debates, neither of the candidates felt he knew what the reaction was to his efforts. The hours after the debate marked the beginning of "spin doctoring." Each side sent its senior staff out to talk to reporters, but, in this case, not even the newsmen claimed they had a feeling about a winner or a loser. On the next day it was the size and enthusiasm of rally crowds that indicated Kennedy had won.

There is another important parallel between 1960 and 2004. Kennedy had to prove that he could stand up to a more experienced vice president, and he did. Nixon's problem was not to look too "tricky." He may have erred by being too deferent to Kennedy.

Vietnam experience will not count when Sen. John Kerry tries to convince the voters that he has the instincts and the experience to handle the next acts of terrorism that may face this country. Regardless of how one judges Kerry's leadership strength, his task is difficult because to be convincing, he must be tough and, above all things, decisive; but if he goes too far, he may gain support from his most ardent followers, but he will lose the all-important undecided voters who will be offended as they were when Al Gore sighed and pushed in the first debate with Bush in 2001.

Bush's challenge will be to avoid being defensive on questions of credibility. How will he answer if he is accused of lying about weapons of mass destruction? Bush needs to keep on the offense, regardless of the questions, and he must avoid seeming "nasty" on subjects such as the "flip flop" issue. He must make the point with dignity or humor. In 1980, Ronald Reagan turned the tide against Jimmy Carter with the quick quip "There you go again."

Both sides will have researchers at work even during the debate verifying comments by their candidate and searching for errors by their opponent.

Candidates have different methods of preparing for the debate. Nixon worked alone with a fact book put together by staff. Kennedy practiced with his staff with what was called the DQ method, "dirty questions" created by Ted Sorrenson and Pierre Salinger of his staff. Reagan originated the plan to practice against an opponent who he believed resembled his adversary. That is the plan being followed by Bush and Kerry today.

In 1960 the hype for the debates came from the TV networks who correctly believed that the debates would give them a breakthrough to become the dominant media in the coverage of politics. Campaign staffs were small in that era. The staffs today are gigantic, and both Republicans and Democrats are using the underdog strategy of football coaches, pointing to the skills of the other side hoping to lower expectations so any good showing will appear better than expected.

Joe Lockhart, a Kerry strategist, told reporters last week that he would "challenge anyone to name a major debate that George Bush has been in where he hasn't been considered the winner." The Bush campaign's top strategist, Matthew Dowd, has described Kerry as being "very formidable and probably the best debater ever to run for president." Dowd later said he was joking, but his point was made.

Bush and Kerry clearly are strong debaters. Bush has the experience of debates for the presidency and in his contests for governor of Texas. He is the more informal of the two and is likely to stick to emphasizing the party line of successes. Kerry started debating in prep school and has 20 years' experience in the Senate. He is likely to be more formal, seeking debating points. Neither will resemble the Lincoln-Douglas style.

In the early days of television coverage of politics, candidates spoke to rallies and occasional press conferences. The first major informal television appeal was made in 1958 when Nixon addressed his TV audience while sitting on the edge of a desk and urged New Yorkers to re-elect Gov. Nelson Rockefeller. The governor was so stunned by the effect of the appeal that he hurried to New York City to be seen having breakfast with the vice president.

The temptation in this week's debate is to speak to reporters or Washington insiders. The candidate who does best will be the one who ignores the Beltway and speaks directly to the voters in the living rooms of America.

This has been the bitterest campaign the nation has seen in modern times, and neither candidate can afford to make a major mistake such as Jerry Ford did in 1976 when he stumbled on the question of Communist control of Poland and then waited days before admitting that his language had been garbled. That may have cost him the close election with Carter. In today's world, such an error would be corrected quickly as a damage control measure.

The format for debates is the result of tough negotiations, and this year the Republicans started out seeking to limit the debates to two and settled for three, plus a vice presidential encounter. The format and central subject for each will vary with foreign policy and national security coming first. The "wild card" will be the second debate where the audience will ask questions. Anything can happen under that format, although those asking questions will have been screened by the Gallup polling organization as "undecided."

Millions will watch Thursday's debate, and some voters who are classified by the polls as "leaning" may change their minds on candidates, but the voters who will make the difference will come from the hotly contested battleground states, ranging in size from New Hampshire to Ohio and Pennsylvania.

The polls that show Bush leading would indicate that this televised hour and a half may provide the most important moments in the long campaign of John Kerry. The challenge is there. Kerry can't afford to break even. He must appear stronger and more clear on his positions than he has thus far in the campaign.

Herbert G. Klein is a national fellow at the American Enterprise Institute.