President Reagan does not plan to respond in detail to the charges of duplicity and illegal activity in his administration aired by the Iran-contra congressional hearings, which are winding down, according to senior White House officials.

Instead, Reagan is planning a speech next week broadly answering some questions raised by the testimony, highlighting procedural changes he has made and asking the nation to look ahead to the remaining 18 months of his term and his "unfinished agenda," the officials said.

That agenda includes a possible arms-control agreement with the Soviet Union, the battle over his nomination of Robert H. Bork to the Supreme Court, additional military aid to the Nicaraguan contras and continued combat with Congress over the budget and taxes.

In particular, White House officials hope to use the speech to buttress what public opinion polls show to be increased support for contra aid as a result of the congressional hearings. "Everyone believes we have a real opportunity on Central America, whether we created it or it was created for us," said a senior White House official.

At a picture-taking session Friday, Reagan once again promised to "speak out" on "this whole affair when the hearings are over," a vow he first made when the hearings began in May. White House spokesman Marlin Fitzwater has repeatedly refused to answer questions on grounds that the president will respond when the hearings are finished.

But there is a consensus among senior White House officials that it would be a mistake for Reagan to try to address the many contradictions and specific questions raised in the hearings. "He recognizes that you can't get in the position of answering each and every little detail," said one official familiar with the president's thinking.

The kind of details Reagan will probably avoid include the circumstances surrounding the secret Dec. 5, 1985, covert action "finding" he apparently signed that explicitly authorized a trade of arms for hostages; the "cover" stories that were developed to hide the administration's role in the November 1985 shipment of Hawk missile parts from Israel to Iran, which may have violated U.S. law; the charges of deceit and subterfuge among Cabinet members last November when the arms deals were first being exposed; and the complex trail of money transactions indicating that intermediaries profited while the contras got only a fraction of the funds diverted from the Iran arms sales.

Nor is Reagan expected to try to clear up the many contradictions in his statements since November, the officials said.

Fitzwater said Reagan does not plan to hold another news conference until this fall, after his 25-day California vacation in August and early September. Reagan has held two news conferences this year, fewer than at any other time in his presidency.

His advisers fear a news conference would draw the president deeper into the details of the Iran-contra affair, while he can use a speech to respond on his own terms. Reagan has also virtually stopped giving interviews, as he once did frequently.

A Republican strategist agreed with this approach, saying that public opinion about the Iran affair has gelled and that further discussion of details by the president "would probably not change anyone's mind." This strategist also noted that Reagan's earlier attempts to deal with the details only plunged him deeper into the contradictions.

Public opinion surveys have shown consistently for the past few months that a sizable majority of Americans believe Reagan is not telling the truth about the Iran affair.

At the same time, White House officials said Reagan has bristled at charges made in the hearings that he attempted to mislead the American people and that he behaved dishonestly. "The part that bothers him more than anything else is the part that he was not honest with the American people," said one of these officials, all of whom asked not to be identified.

The president also resists suggestions that subordinates may have broken the law. "I haven't heard a single word that indicated in any of the testimony that laws were broken," he said at Friday's picture-taking session.

The officials said they were undecided about whether it would be worthwhile for Reagan to address the credibility problem one more time in the speech or ignore it. They said a draft of the address has not yet been sent to the president and that the approach may change as a result of further discussions this week.

Reagan's anger and frustration at doubts about his truthfulness spilled into the open last week when Fitzwater accused "some members of the press" of trying to "destroy" Reagan over the affair. Fitzwater commented after speaking to a visibly agitated president about news accounts of remarks the president had made at a key meeting last November urging subordinates to refrain from talking about specifics. "Don't talk TOWs, don't talk specifics," Reagan said then, according to notes taken by then-deputy national security adviser Alton G. Keel Jr. and made public by the Iran-contra panels.

Reagan has maintained he concealed information only to protect the lives of the hostages. Other notes released last week show that some Reagan subordinates believed they could get still more hostages out by trading more weapons if details were kept secret. At the same meeting, then-chief of staff Donald T. Regan noted, "Can continue to get hostages {out}."

White House officials said "a good benchmark" of how Reagan will respond to the hearings was the March 4 address he gave following publication of the Tower commission report. The commission sharply criticized Reagan's management style and provided new details of the Iran deals. At that time Reagan said "I accept" the critical findings and said the initiative "deteriorated" in trading arms for hostages, but he defended his motives and stressed changes in personnel and procedures he had made.

White House officials and Republicans outside the administration said there had been conflicting opinions about the tone of the speech which will be delivered no later than Aug. 12. One longtime conservative Reagan adviser not part of the current White House team said, "I don't know that I would go on the air after the hearings," adding that he would just ignore them.

However, White House officials said Reagan is committed to the address. They predicted he will adopt a tone that one described as "healing" rather than confrontational.

They said Reagan hopes to capitalize on the polls showing increased support for military aid to the contras, which in the past has faced sizable public opposition. "The hearings have done more to expose the policy, as opposed to the implementation, than we ever could have imagined," said a White House official. "Now we say to Congress, 'you've had your fun, there were problems with the implementation, but focus on the policy, and we've got to keep it on the policy.' "