ATLANTA -- In the long, tough and immensely healthy contest that lies ahead before the presidential nominations of both parties are settled, the candidates who will prevail are those who can best deal with the unremitting pressure.

The good news for the country is that several of the contenders already have demonstrated the required mental and emotional toughness. The better news for the reporters lucky enough to be covering this fascinating campaign is that we don't know which of them will ultimately prevail.

If tradition holds, either George Bush or Michael Dukakis will be the next president of the United States, because no one has gained that office since 1952 without first winning the New Hampshire primary. But anyone who ignored the other contenders would be foolish. Richard Gephardt and Robert Dole, the midwestern runners-up to Massachusetts-born Dukakis and Bush, are very much in the running. And others cannot be counted out.

Bush has shown that he can take a hard political blow -- like the third-place finish behind Dole and Pat Robertson in Iowa -- and still recover. It was stunning to see the contrast between Bush's post-Iowa resiliency and the raw emotions Dole displayed in his frustration when a tantalizing New Hampshire victory slipped away.

Dole's outburst in the Tuesday night interview with NBC's Tom Brokaw, when he told Bush to ''stop lying about my record,'' may be as damaging as the defeat itself.

The group of Republican voters assembled by The Washington Post who watched the Sunday night League of Women Voters debate tipped us to the coming Bush ''upset.'' They liked Bush's performance and were deeply disturbed by Dole's needling comments and sarcastic asides to almost all his rivals during the hour on stage at St. Anselm's College.

My theory of what happened in New Hampshire, for what it is worth, is that the dynamics of the Republican race essentially repeated those of 1980. The Post was in the same community, with some of the same Republican voters, eight years ago when Ronald Reagan was struggling to shake off an Iowa defeat at the hands of Bush.

That night, too, the League of Women Voters debate seemed a no-decision to most of the reporters on the scene. But our living-room viewers unhesitantly called Reagan the winner and so he proved to be.

The reason was simple: they had been predisposed to vote for Reagan after watching his good losing campaign in their state in 1976. But they had been jarred by reports that he seemed old, listless and ineffective in his losing effort in Iowa that 1980 winter. When they saw him at least hold his own with his rivals in the League debate, that was all the reassurance they needed -- and they went back to their original preference.

In my view, that is what happened with Bush this year. As Reagan's vice president and a familiar figure to New Hampshire Republicans, he was their ''natural'' choice. But they needed to see he was the same George Bush they had known -- not the Iowa scarecrow they had heard about. Once they saw that, on stage at St. Anselm's and in the hastily prepared Bush programs and commercials that flooded television, they started coming home.

Dole's nastiness and the powerful grass-roots organization Gov. John Sununu put at Bush's disposal helped turn what Bush aides acknowledge as a looming defeat last Thursday into a handsome comeback win on Tuesday night. But credit Bush's tenacity for being the key.

On the Democratic side, there were two winners -- not one. Dukakis ran impressively, carrying both blue-collar and upscale Democratic constituencies as he avoided the capriciousness that had seen New Hampshire voters humble Ed Muskie of Maine and reject Ted Kennedy of Massachusetts in previous primaries. For those who know the state's history, his 16-point victory margin says, ''This man is a serious player, in for the long run.''

But Gephardt's second-place finish may send an even stronger message about who has the toughness and tenacity to go the distance in this race. While Dukakis sailed through the contest unscathed, the young Missouri congressman took the worst pounding of his political career during the week of the New Hampshire primary. He was hammered for his ''inconsistency'' in biting commercials from rival Paul Simon, was gang-tackled in the Democratic debate by three of his opponents, Simon, Albert Gore Jr. and Bruce Babbitt, and was the subject of uncounted skeptical television and newspaper pieces reexamining his voting records.

Except for one news conference late Friday evening, when his frustration made him sound petulant, Gephardt never wavered under the assault. And by Tuesday, he was again, as he had been in Iowa, the best campaigner in the state -- carrying a message of Jesse Jackson-like intensity to a broader constituency than Jackson has yet shown he can reach.

The South responds to strong campaigners; the reflex is part of the region's heritage and character. Dukakis and Gore have more money and organization than anyone else ready for Super Tuesday. But the campaigners are Gephardt and Jackson, and the implications of that fact should not be forgotten. "Credit Bush's tenacity for being the key. . . . Gephardt never wavered under the assault."