In less than a quarter-century, televised presidential debates have taken root as a staple of modern politics -- short on rules and tradition, long on stress for the candidates -- a quadrennial "High Noon" for the Electronic Democracy.

The three rounds of showdowns thus far, beginning with the Kennedy-Nixon debates in 1960, have helped tumble incumbents and elevate underdogs.

The public likes them. About 100 million Americans, or about 40 percent of the population, watched the last such encounter four years ago between President Jimmy Carter and Republican challenger Ronald Reagan.

The results most often have turned not on factual points scored but on revelations of the human dimension as filtered through the small screen -- Richard M. Nixon's "5 o'clock shadow," for example, and Reagan's avuncular "There you go again."

Critics have slammed the genre for not being "debates" at all, but staged performances whose importance has been inflated beyond reason.

For better or worse, however, debates have become a focus of drama and suspense for the campaign season.

In 1980, just before the debate in Cleveland, public support had swung sharply toward Carter, indicating that he had succeeded in raising doubts about Reagan's ability to keep the country out of war and in bolstering public confidence in his own ability to deal with the economy.

But after the Oct. 28 debate, Carter's support faltered and never recovered. Reagan's election victory was the result of "one of the most dramatic shifts ever recorded in voter preferences in the last week of a presidential campaign," according to pollster George Gallup Jr.

No debate had ever been held so close to an election -- just one week before voters went to the polls. National surveys taken immediately after the debate showed that most voters who had watched thought Reagan had won. But it was unclear whether Reagan had gained voting strength, since the viewing audience included a disproportionate number of people who already supported him.

One poll showed that both candidates gained equally, and another showed that Reagan had gained only slightly more than Carter.

What people seemed to remember most about the encounter was the way Reagan caught Carter offguard at the outset, bounding across the stage to clasp the president's hand in greeting; and Reagan's patented "There you go again" retort to a Carter criticism of his views on Medicare; and probably most important, Reagan's closing question pointing up concerns about Carter's economic record: "Are you and your family better off than you were four years ago?"

People also remembered Carter's comment on defense policy, which Republicans lost no time ridiculing: "I had a discussion with my daughter, Amy, the other day before I came here, to ask her what the most important issue is. She said she thought nuclear weaponry and the control of nuclear arms."

The debate was held at Cleveland's Convention Center and sponsored by the League of Women Voters. The candidates were questioned by Barbara Walters of ABC; Marvin Stone, editor of U.S. News & World Report; Wiliam A. Hilliard of The Oregonian newspaper in Portland, Ore., and Harry Ellis of The Christian Science Monitor, with Howard K. Smith as moderator.

If there is an established axiom about these unpredictable face-offs, it is that a challenger such as this year's Democratic nominee, Walter F. Mondale, benefits just by appearing on the same stage with an incumbent, thereby taking on the aura of an equal. For a challenger, a tie with an incumbent president is said to constitute a "win."

The presidential debates of 1976 seemed to halt the momentum of another incumbent president, Gerald R. Ford.

Carter, as the challenger that year, had been leading by as much as 18 percentage points in the polls in early September, but by the eve of the debates, the contest between the two men was a dead heat.

During the first of their three face-offs, the sound system failed with eight minutes to go, and the president and his opponent were forced to wait on their feet at the lecterns for 27 minutes while the engineers repaired it.

In the second debate, Ford stated that "there is no Soviet domination of Eastern Europe" and added that he did not believe that the people of Yugoslavia, Romania and Poland "consider themselves dominated by the Soviet Union." With this, followed by his nearly week-long refusal to recant, the Democrats were able to portray the president as inept.

The Ford-Carter debates were held Sept. 23 in Philadelphia, Oct. 6 in San Francisco and Oct. 22 in Williamsburg. Among the panelists were Edwin Newman of NBC, Frank Reynolds of ABC, Elizabeth Drew of The New Yorker, Barbara Walters of ABC and columnist Joseph Kraft.

The vice-presidential candidates that year, Republican Sen. Robert J. Dole of Kansas and Mondale, then a Democratic senator from Minnesota, debated each other Oct. 15 in Houston.

Mondale angered Dole by bringing up the Watergate scandal that ended the Nixon administration and promoted Ford to the presidency. Dole countered with reference to the 1.6 million Americans "killed and wounded" in what he called "Democrat wars in this century." Mondale, later deemed the winner of the exchange, called Dole a "hatchetman" and asked, "Does he really mean that there was a partisan difference over our involvement in the fight against Nazi Germany?"

Carter said after winning his close election victory -- he had 51 percent of the popular vote to Ford's 48 percent -- that he never would have made it to the White House but for his three debates with Ford.

The 1960 debates between Vice President Nixon and Sen. John F. Kennedy of Massachusetts ushered in the era of television politics. An estimated 70 million viewers tuned in to watch the spectacle in grainy black and white.

Nixon, as Dwight D. Eisenhower's vice president the better-known of the two, had the edge in the polls before the first debate. But the public awarded Kennedy the victory, by 2 to 1, and Nixon fell into a permanent trailing position.

In one of the all-time squeakers, Kennedy won 49.7 percent of the popular vote to Nixon's 49.5 percent, according to Congressional Quarterly.

Theodore H. White, in his book "The Making of the President, 1960," argued that Kennedy won the election in his first debate with Nixon. On that occasion, Nixon refused to be made up for television, except for a cosmetic powder called "Lazy Shave," designed to hide the heavy stubble on his chin. The powder streaked, however, when he perspired under the lights.

A Roper poll found that 57 percent of that year's voters said they were influenced by the debates and that 6 percent, or 4 million, said they based their decision on the debates. Of that total, Roper said, Kennedy won 3 million.

A major issue in the 1960 debates was the defense of Quemoy and Matsu, two Nationalist-held islands near the Chinese mainland. But few people who saw the debates remembered what the candidates said about that.

What they did remember was Kennedy's bright-eyed poise and aggressiveness in contrast to Nixon's haggard, shifty-eyed and sweaty appearance. Those who watched the debates thought Kennedy had won, while those who only heard them on radio thought Nixon had.

The four debates took place Sept. 26 in Chicago, Oct. 7 in Washington, Oct. 13 with Nixon in Los Angeles and Kennedy in New York, and Oct. 21 in New York. Among the panelists were Howard K. Smith of CBS, Sander Vanocur of NBC, Roscoe Drummond of The New York Herald-Tribune, Walter Cronkite of CBS and John Chancellor of NBC.

Sixteen years would pass before the next televised presidential debate.

Presidents Lyndon B. Johnson in 1964 and Nixon in 1968 and 1972 -- the latter still smarting from his 1960 experience -- refused to debate their opponents. Both thought, correctly, that their leads were sufficient that they could get away with not debating.

But willingness to debate in itself can affect a candidate's standing in the polls, and only the most secure can remain above the fray. The fact that Reagan, despite his lead in the polls, has agreed to debate Mondale may set a precedent that will make it harder for future front-runners to avoid meeting their challengers face to face.

In 1980, President Carter scorned entreaties to debate his challenger in the primaries, Sen. Edward M. Kennedy of Massachusetts, and later rejected a three-way debate that was to include independent candidate John B. Anderson, agreeing only to a main event with Reagan. Polls showed that most people favored a three-way debate.

The sponsoring League of Women Voters considered placing an empty chair on stage with Reagan and Anderson as a rebuke to Carter. The league's members thought better of it, but not before the idea prompted criticism of them and a joke from comedian Johnny Carson, who wondered, "Suppose the chair wins?"

This was the first time the league had been forced to deal with the issue of whether a non-major-party candidate should participate. It drew harsh accusations from Anderson supporters that it had capitulated to the Carter White House when it later decided that Anderson's support in national polls had slipped to the extent that he did not qualify as a serious contender and would be excluded from the Reagan-Carter debate.

In a technically messy attempt at media one-upmanship, however, the Cable News Network gathered Anderson and an audience at Constitution Hall here and spliced his statements and rebuttals with those of Reagan and Carter as the main event was occurring in Cleveland.

Also in 1980, candidate Reagan refused to appear with other Republican contenders in an Iowa debate and suffered from his decision. He learned from the experience and regained ground in Nashua, N.H., where he dramatically opposed the exclusion of fellow Republicans from a debate that his campaign was paying for. The sponsoring newspaper and George Bush argued that they should be barred.

When Jon L. Breen, executive editor of the Nashua Telegraph, urged someone to "pull the plug on Gov. Reagan's mike," Reagan furiously grabbed the device and snarled, mispronouncing the name, "I am paying for this microphone, Mr. Green."

The most memorable prototype of the modern presidential debate is, of course, the series of seven formal debates between Republican Abraham Lincoln and Democrat Stephen A. Douglas in their 1858 race for an Illinois Senate seat.

Lincoln lost the race but made enough of a name for himself to run successfully for president two years later.

Each Lincoln-Douglas debate was held in a different Illinois congressional district, starting in the town of Freeport. One candidate would open with a one-hour statement, the other would respond for an hour and a half, and the first would return for a final give a 30-minute rebuttal. While Douglas' voice was said to be deep and sonorous, Lincoln's was described as high-pitched and loud. Both were known for their quick wit.

Their debates required the audiences to stand in outdoor settings for long periods of time. However, historians note that the two men were actually heard by more people in their individual campaign appearances than in their joint ones.

The debates subsequently were printed in the leading newspapers of both parties and as a book, which became a focus of the 1860 presidential campaign.

One of the trickier aspects of the modern debate game is that it requires the opposing camps to get together and order to negotiate a complex package of dates, formats, settings and other compromises that will offer each side the hope hopes will give it the advantage over the other. For example, this year, Reagan's aides agreed to two 90-minute rather than 60-minute debates in exchange for an agreement that Reagan could use a special hearing amplifier.

The television networks have also become players in the political drama. In 1976, network officials at first threatened not to cover the events because of a restriction, agreed to by the candidates and the sponsor, that the networks would not be allowed to televise audience reaction. They later backed down.

But the "debate debate" continues each election year, with the various parties arguing about what form the events should take. Should they be real debates in which the candidates challenge each other directly, as in some of the primary debates earlier this year, or must they be the rigid, news-conference-like productions that the presidential formats have become, with a panel of journalists asking the questions and setting the tone?

The League of Women Voters, now apparently ensconced as the official sponsor of the presidential debates, was drafted to hold the debates in 1976 when the TV networks were forced by the Federal Communications Commission's "equal-time" rule to present the spectacles as news events rather than productions staged for television. so as not to violate "equal time" requirements under Federal Communications Commission rules For the 1960 debates, Congress had temporarily suspended the equal-time provision.

There are some newcomers getting into the act, at least in the early eliminations. The Des Moines Register and the Nashua Telegraph, which got into the game by sponsoring Republican forums in 1980, did it again last winter for the Democratic contenders.

If the ground rules for the debaters are still developing, so are the methods of scoring. Political-entrails readers have not yet found a reliable method for rapidly and correctly analyzing the impact of these events.

The day after the 1980 debate, for example, one Washington Post reviewer wrote that if the debate were "the only presentation of issues and personalities" in the campaign, "Jimmy Carter would probably walk away with all the moola . . . . He won by making Ronald Reagan look flustered and bothered as he tried to defend his position."