MANCHESTER, N.H. -- A useful way of analyzing the situation of the 13 presidential hopefuls, in their passage from Iowa to New Hampshire, is to ask which of them must now change their messages and which need not bother. That perspective provides a different lineup of winners and losers from the Iowa results.

To dispose of the easiest questions first, the quartet of ''formers'' -- ex-senator Gary Hart, ex-governors Bruce Babbitt and Pete du Pont and ex-secretary of state Alexander Haig -- don't have to do a thing. As weak finishers in Iowa, with no better prospects here and no funds to sustain their campaigns into March, they can say whatever they like for the next few days, because it won't make much difference.

Nor do the two reverends need to change their scripts. Pat Robertson and Jesse Jackson both did well in activating constituencies in Iowa while erasing some of the tinge of extremism from their images. In some ways, Jackson's fourth-place finish in the Democratic race in a state with very few blacks was more striking than Robertson's more publicized feat in finishing second in the GOP contest with the available fundamentalist votes.

Both of them are pointed South for the March 8 ''Super Tuesday'' contests and both clearly will be major factors there.

Two others who do not have to change their scripts are Sens. Robert Dole, the winner of the Republican primary, and Albert Gore Jr., the last-place finisher among the Democrats. Dole's win was not just a gesture to a Kansas neighbor. He has become the best of the Republican campaigners, perhaps the only one consistently offering a fairly full (if self-flattering) picture of himself and of his approach to the presidency and some of the nation's major issues. For a man with major insecurities as a presidential candidate, Iowa had to be a huge confidence-builder. And a confident Dole will be a formidable contender.

Gore did the opposite, avoiding Iowa entirely. And this week he will be only a blip on the New Hampshire screen. The Tennessean can take comfort from the fact that New Hampshire is likely to reshuffle the order of finish among the three closely bunched top Democratic candidates in Iowa -- Rep. Richard Gephardt, Sen. Paul Simon and Gov. Michael Dukakis -- and thus deny any one of them great momentum coming into the South on March 8. But Gore has left himself only one shot at getting into the race.

Two candidates who probably can't change their messages, even if they want to, are Simon, the Illinois Democrat, and Rep. Jack Kemp, the New York Republican. Kemp has been moving up in New Hampshire by arguing that he -- unlike Vice President George Bush and Dole -- would not reduce Social Security benefits, raise taxes or trim a dollar from defense. Voodoo economics is still his religion. His financial plight requires that he beat out Robertson for third here, which he should be able to do, and crowd at least one of the two favorites, which may be harder.

Simon's old-time Democratic religion has won him a following in this state, and he would jeopardize it if he changed tone at all. But he cannot survive another beating by Gephardt here, and he is in danger of getting one.

As for Gephardt, he developed into the best Democratic stump speaker except Jackson during the final month in Iowa and managed an amazing transformation of character and message. Somehow Gephardt, the consummate congressional insider, was able to fly a planeload of fellow congressmen and business lobbyists to Des Moines to endorse him and still present himself to voters as a scourge of corporate America and the establishment.

Having managed that feat, it should be easier for him to turn back to his true self -- a moderate Democrat with strong back-room negotiating skills -- as the climate of New Hampshire and the South dictates. But his rivals may not be as forbearing of his flip-flops as they were in Iowa.

As for Dukakis, it would take a major blunder for him to lose New Hampshire to a divided opposition, but his message problems are real. He has yet to find a way to communicate the genuine passion he feels about the stagnant parts of urban America and the people trapped in them. And his approach to foreign policy comes so close to renouncing the unilateral use of American power to protect national interests, while relying on international covenants and organizations that have looked futile for 40 years, that it sometimes sounds as if the ghost of Eleanor Roosevelt had taken control of his body.

But such problems seem minor compared with those of vulnerable Republican front-runner Bush. Eight years ago he came into New Hampshire as the winner of Iowa and lost here because he had nothing of substance to say to people.

This year, he comes in as the loser in Iowa, and he still has nothing of substance to say. Loyalty to Reagan and a vague commitment to be ''the education president'' will help in this state, but they are not enough to sustain Bush for long. He really needs to think why he wants to be president -- and let voters in on the secret.