His emotion showing, President Carter last night defended his decision to order the aborted rescue mission into Iran and told the American people that its failure paled next to what would have been "a deeper failure -- the failure to try."

In a nationally televised news conference, his first since the rescue operation ended abruptly with the deaths of eight American servicemen in the Iranian desert, the president said in the aftermath of the attempt that "there is no guilt I feel on behalf of our nation."

Carter said, in response to a question, that throughout the Iranian crisis political considerations "have not been a factor for me," that he has had "to make decisions that proved to be unpopular . . . in the most difficult of circumstances."

But in a broader sense, his statement opening the news conference and his response to questions about the rescue mission that dominated the 30-minute session amounted to a political defense of the most desperate act of his presidency.

Using a series of emotion-laden words, the president described the display of the bodies of the dead American servicemen at the U.S. Embassy in Tehran as a "ghoulish action of terrorists, a powerful exhibition of inhumanity that has aroused the disgust and contempt of the rest of the world."

This "desecration," he said in an explanation of what led him to order the rescue mission while knowing it would result in the resignation of Secretary of State Cyrus R. Vance, is "an indication of the kind of people they [the Iranians] are, a vivid indication of the difficulties we have encountered" in seeking a negotiated settlement of the crisis.

Carter returned repeatedly to this theme of the difficulty of negotiating with "the type of people we have been dealing with" in an apparent appeal to Americans to understand the six months of diplomacy that preceded the operation and the decision finally to gamble on the high-risk attempt to free the American hostages by force.

There was nothing in what the president said to suggest much hope that further negotiations would chance the pattern of frustration that has marked earlier attempts to gain release of the hostages peacefully. This impression was reinforced when Carter announced, without elaboration, that when he met secretly Sunday with members of the rescue team at an undisclosed location, they had requested "to be permitted to try again."

The president, for the first time, also disclosed that a rescue attempt, rather than a larger military maneuver designed chiefly to punish Iran, would have been the U.S. response had any of the hostages been harmed by the Iranian militants holding them. He said planning for the mission had begun in November, shortly after the embassy takeover in Tehran, and that the operation "would have had to be undertaken had the hostages be injured."

But while Carter was making this defense of his handling of the crisis largely for domestic consumption, he renewed his appeal to Iranian authorities to end the stalemate for the good of their own country.

Asserting that he has no doubt his decision to order the rescue mission was "the right decision," the president said:

"It would have ended a continuing crisis that is destabilizing for the people of Iran, causing them immense political and economic damage . . . and it would have made unnecessary the upcoming sanction which will be much more severe."

This was a reference to the economic sanctions that U.S. allies have pledged to impose by mid-May.

With the crisis ended, Carter said, "we could have begun restoring Iran as an accepted nation in the world community." The continuing state of chaos and crisis, he said, has denied Iranians the progress they hoped to achieve through the revolution that overthrew the shah.

"They have not even been able to hold an election," the president said.

Throughout the news conference, Carter's personal emotion over the events of the last few days was evident. At times he seemed to gasp for breath as he spoke about the rescue mission and its aftermath, which accounted for 14 of the 15 questions asked him.

Like other administration officials, the president refused to discuss details of the operation beyond the point at which it was terminated. But he insisted that while it involved "some risk" it had "a good chance of success." He repeated his earlier statement that the decisions to launch and terminate the mission were his, but noted that the commander in the field and his military advisers concurred in his order to end the operation.

He also said there was "no connection" between the mechanical problems in three U.S. helicopters that caused the termination of the mission and the general problem of retaining skilled personnel in the military services.

Asked if the nation's honor now took precedence over the lives of the hostages, Carter said quietly, "No," and went on to describe the preceding months of diplomacy.

"We had exhausted every peaceful procedure, we could not have waited much longer," he said.

In response to another question, the president denied the growing assertion that he has allowed the hostage crisis to preoccupy his administration to an unhealthy degree. He cited other administration activities over the last six months, but conceded that he shared "the human kind of concern" that has marked the prolonged struggle over the hostages' fate.

Carter opened the news conference with a statement on the rescue mission. He said he shared the nation's disappointment over its failure, grief over the casualties and pride in the men who attempted it.

Contrasting the behavior of the American rescue team with that of the Iranian militants, he said, "Our goal is not to conquer, their goal was not to destroy or injure anyone."

The president said the United States "will try for a peaceful resolution" to the crisis, but added, "We will not forget our hostages."