Presidential debates are survival contests, and tonight George Bush was the co-frontrunner who emerged unscathed and unscarred after a relatively gentle debate.

For one hour and 15 minutes, Ronald Reagan, the other Republican who is running well ahead of the presidential pack in all of the New Hampshire polls, was doing just as well as Bush, handling all the questions that came his way in a forum discussion that was heavy on issues and virtually devoid of intraparty party.

But when the last questioner of the evening moved to the audience microphone, identified himself as an Italian-American and asked about Reagan's recent ethnic joke about Poles and Italians, there was little Reagan could say that would ward off political damage.

"I'm glad you asked that question," Reagan said -- but his somber expression made clear he was not. Running virtually even with Bush, Reagan knows he needs to score decisively in the Manchester area, which is his stronghold, if he is to win the presidential primary next Tuesday. Manchester has a sizable Italian-American population. It did Reagan no political good to have them reminded of his recent joke. And his answer tonight that "I had not told the story the way it was reported . . . I was the victim . . . I was stiffed" was lame -- although there was probably no good way out.

The questioner was Vincent Galati, a stockbroker from Durham who said he voted for Reagan in the 1976 primary but won't this time.

Tonight's contest was approached by both Bush and Reagan as one in which they had to, at the very least, protect their frontrunner positions from the rest of the pack. Before the debate, Bush told The Washington Post in an interview, "I don't think it [the debate] is something that can be won. I think it is something that can be lost if someone makes a big mistake."

As the forum turned out, neither Bush nor Reagan was significantly challenged by any of their candidate-colleagues and neither of them -- nor any major candidate on the panel, for that matter -- committed a major mistake. a

Instead, the performance on the auditorium stage in Central High School resembled a Grand Old Party septet, long on harmony, frequently in unison, with few discordant notes.

Sen. Howard Baker of Tennessee made several attempts to draw gentle distinctions between his views and those of Bush, who has been winning much of the center-to-moderate support that Baker had once hoped would be his. But Baker's efforts probably made little impact on voters already leaning toward Bush.

When Bush sounded the standard line of presidential challengers from both parties that the Soviets must have been "confused" by President Carter's shifting emphasis in foreign policy, Baker, who spoke next, countered: "I don't think the Russians are confused -- I think they think we are a patsy."

Later, Baker took issue with Bush's position in favor of indexing taxes, and said that Bush opposes a constitutional amendment to prohibit deficit spending while "I favor it."

But Baker's remarks, delivered in the gentle and gentlemanly ways of the U.S. Senate, were not the sort that would be remembered by viewers.

For Ronald Reagan, up until that last question about his ethnic joke, the debate was a mild rhetorical and political success. He had angered a number of voters in Iowa by refusing to debate there just before the caucus voting in which he was defeated by Bush. But tonight he showed that he could indeed debate the issues in the low-keyed manner adopted by his colleagues, handling himself sufficiently well on all of the issues and responding no more or less memorably than all the rest.

One other fact of life of the televised debate worked against Reagan, however. The harsh realty of the camera showed Reagan clearly the eldest of the pack, his face creased by the lines of 69 years even though his hair remains dark and full, a sort of wetlook version of George Bush's.

In all of the polls -- most recently a nationwide poll released by The New York Times and CBS -- Bush has shown a large base of popularity, while maintaining a very low negative rating. Political advisers view a low negative rating as a decided blessing for a candidate, and nothing tonight should worsen it.

The debate, by and large, steered clear of the highly emotional, negatively charged issues that Reagan's supporters have been trying to use against Bush. At one point these issues -- gun control, abortion and prayer in schools -- were specifically cited in a question to all candidates. But Bush and Reagan both handled the question without damaging themselves.

Both frontrunners prepared for the debate, according to their advisers, sessions designed to ready them "psychologically" (Reagan and Bush aides both used the term) as well as factually.

Reagan advisers were concerned primarily that he be able to speak precisely on the issues in the limited time available. "He tends to give long answers," said a top assistant. By and large, Reagan handled the time limitations well.

Bush's advisers said they were concerned that their candidate might not fully appreciate the event. One aide pointedly told Bush in his briefing that he had not fully utilized the time available in the Iowa debate. Tonight, Bush seemed better prepared in outlook as well as content.

Bush wore his own campaign button in his left lapel, the only panelist so adorned.

In the end, in an exercise where the goal is cutting losses and avoiding mistakes, Bush survived the fittest.