Secretary of Defense Caspar W. Weinberger, explaining yesterday why he opposed from the outset the Reagan administration's secret plan to improve relations with Iran, called the initiative a bad idea based on a faulty premise that a "moderate" element existed within the Islamic revolutionary government.

"I didn't think there was anyone we could deal with who was not virulently anti-American," Weinberger told the Iran-contra congressional committees in his first day of testimony. "I did not think and do not think there's any moderate element that is still alive."

He faulted the National Security Council staff for stringing the president along with favorable reports about the initiative, which he thought he had managed to kill several times only to find that it had been revived.

He singled out former national security adviser John M. Poindexter for special criticism, saying that Rear Adm. Poindexter promised to obtain release of all hostages with the first shipment of arms before allowing any more weapons to be sent.

Weinberger followed former White House chief of staff Donald T. Regan at the witness table, beginning the final hours of public testimony that began 11 weeks ago. Once Weinberger concludes his testimony, probably Monday, the committees will hold several days of closed sessions with witnesses from the Central Intelligence Agency.

In other developments yesterday:Regan, appearing for several hours after a full day on Thursday, confirmed previous reports that the president was asked last December about the possibility of a pardon for Poindexter and former White House aide Lt. Col. Oliver L. North. But, he said, "it got shot down right away."

Regan said, "That was something the president wouldn't even listen to . . . . To grant a pardon means you think somebody's committed a crime. You only pardon for a crme. And {the president} didn't know what the crime was. As yet there had been no evidence brought to him." Sen. Sam Nunn (D-Ga.), referring to Regan's handwritten notes of a high-level meeting with the president last Nov. 10, had a sharp exchange with Regan over what led the president to say during the meeting that Iran was the "weaker" side in the Iran-Iraq war.

Nunn, who chairs the Senate Armed Services Committee, said, "That was not the position of our government. Our government's position -- and I went over this with Secretary {of State} George {P.} Shultz {when he testified} -- was that the Iraqi side was deteriorating, and that the Iranian side had the long-range advantage . . . . " Regan defended the president, at one point saying he could not be specific without disclosing classified material. Rep. Les Aspin (D-Wis.) disclosed that the Kuwaiti foreign minister, relying on his intelligence information, asked Deputy Secretary of Defense William H. Taft IV last October if the United States was selling arms to Iran. Taft, unaware of the secret shipments, said no. Weinberger revealed that he first learned that U.S. and Iranian officials were negotiating potential arms sales in 1985 when he received "a series of intelligence reports." Later, he found out that he was not supposed to have seen the reports, which came from an intelligence "agency" under his control, a reference to the National Security Agency.

That angered him and he instructed his military assistant to remind the agency that it was under his control.

Weinberger argued in meetings before the Iran initiative began and after it was disclosed that selling arms to Iran exposed the United States to "blackmail." He told top White House officials at a Nov. 10, 1986, meeting, "We must bear in mind we have given the Israelis and the Iranians the opportunity to blackmail us by reporting selectively bits and pieces of the total story," according to a memorandum he wrote to the file after the meeting.

Weinberger's testimony in opposition to the arms-for-hostages dealings with Iran echoed similarly negative and critical assessments from Regan and Shultz. Taken together, the statements of the three appeared to write the final epitaph to the controversial policy.

Weinberger added a new element yesterday when he took issue with one of the rationales used by Poindexter to brief Reagan last Jan. 17, and later employed by the president to justify the initiative.

This view, which suggested that Iran was the "weaker" side in the Iran-Iraq war, and thus needed U.S. weapons in 1986 in order to maintain "balance" in the conflict, was expressed by Reagan at the Nov. 10 meeting at which Regan took his handwritten notes. That session with top officials was called to draft a public statement explaining the sales that had just become the subject of sketchy news stories.

Poindexter initially put forward the idea that the Iranian position in the war was "deteriorating," and attributed this view to the Israelis. This position was included in Poindexter's Jan. 17, 1986, cover memo attached to the intelligence "finding" in which Reagan authorized the covert initiative with Iran, including the 1986 arms transactions with that country.

"I certainly did not have the view that Iraq was winning or anything of that kind," Weinberger said yesterday. "Quite the contrary." He said he would have expressed his disagreement "in the strongest possible terms" if he had known of the memo.

Nonetheless, last Nov. 10 neither Weinberger nor Shultz, who also opposed the policy and differed with the view that Iran was "weaker," spoke up at the meeting to correct the president, Regan testified. Weinberger, when asked about it yesterday, said he could not recall the president making that statement at the meeting.

Weinberger, however, did lay out for the committees how he registered his strong disapproval of the Iran initiative at many key points along the way, as the policy grew from U.S. acquiescence to Israeli shipment of U.S.-made arms in mid-1985, to a full-blown U.S. covert action directed by the National Security Council with support from the CIA in 1986.

"I can't think of anything else that might have been done that would have been in any way effective," he said. "As we lawyers say, you run out of appeals after a time . . . . I was not able to be persuasive enough and I'm sorry that I wasn't."

Before Weinberger's appearance yesterday, the Reagan assessment of a "weaker" Iran triggered an exchange between Regan and Nunn.

Nunn wanted to know why Shultz, Weinberger or other officials at the Nov. 10 meeting had not corrected the president.

"Here you had the president of the United States giving what is essentially erroneous policy, a policy statement. It wasn't small detail. It was a question of who our government believed was winning the war . . . and we have classified reports from the whole community, including intelligence, including defense, including state, saying exactly the opposite of this, and the president makes this statement, which is obviously fundamentally conflicting with the policy of our, supposed policy, of our government, and nobody corrects him. Is that what happened?" Nunn asked.

When Nunn pressed Regan on whether he believed the Iranian position was deteriorating and Tehran was losing the war, Regan assumed his once familiar role as loyal defender of the president, indicating somewhat mysteriously that he could not answer fully because it might give away classified information.

"I won't push Mr. Regan on this," Nunn said. "But it is not classified that the United States government position at that time was that we felt the Iraqi position was deteriorating -- that's not classified."

Outside the hearing room, Nunn has suggested previously that the disclosures of U.S. arms sales to Iran resulted in the Kuwaiti overtures to Moscow for protection, which in turn spurred the Reagan administration to protect Kuwaiti shipping in the Persian Gulf under the reflagging arrangement.

This theme was echoed yesterday during Weinberger's interrogation by Aspin, chairman of the House Armed Services Committee. Weinberger denied this, saying, "I am absolutely convinced there was no connection . . . and if you'll forgive me for saying {it}, even the attempt to establish a connnection is fairly tenuous."

Nunn also called the reflagging a "tilt toward Iraq."

Before Regan completed his testimony, several committee members again praised his candor, as some had done yesterday, and Sen. Howell Heflin (D-Ala.) suggested that the Tower commission had done Regan a "disservice" in its report on the causes of the Iran-contra affair.

The Tower report concluded that Regan, because he was chief of staff, "must bear primary responsibility for the chaos that descended upon the White House" after the arms sales became public.

Regan repeatedly stressed in his two days of testimony that he had argued for getting all the facts out last November and that he was often in the dark about the Iran initiative and other activities of the NSC staff, which he emphasized was not under his control or supervision.

Sen. Paul S. Sarbanes (D-Md.), adopting his theme, declared, "You, in effect, were frozen out there by Adm. Poindexter."

Sarbanes referred to a computer message that North sent Poindexter, which said, "I have no idea what Don Regan does or does not know re my private US operation {to aid the Nicaraguan contras}." Poindexter replied, "Don Regan knows very little of your operation, and that is just as well."

"It seems to me," Sarbanes went on, "what happened in all of this is in effect there was a junta within the government of the United States. I mean, you have Poindexter and North and their associates in effect invoking the authority of the president of the United States, freezing out the chief of staff, the secretary of defense and the secretary of state and pursuing their own policy."

Sarbanes later said that "a coup in effect had occurred in the White House," in which Regan was "left there with this having happened under and around him . . . unware that it was taking place."

Sarbanes picked up the theme later with Weinberger, referring to another computer message in which Poindexter told North, "I don't want a meeting with Ronald Reagan, Shultz or Weinberger," on the eve of former national security adviser Robert C. McFarlane's May 1986 trip to Tehran for continued negotiations.

"Senator, what was taking place I believe . . . was that people with their own agenda who thought this opening was a good thing, who knew that I opposed it, that George Shultz opposed it, did not want the president to hear these arguments after the decision had been made," Weinberger said.

In answer to a string of questions from Sen. James A. McClure (R-Idaho), Regan said that "as far as I know," Reagan did not give advance approval for Israel to transfer 500 U.S.-made TOW missiles to Iran, and provided no assurances that the missiles would be replaced by the United States if they were delivered.

McClure referred to Regan's handwritten notes of the Nov. 10 meeting of the president and his top national security advisers to discuss the Iran situation, in which Shultz is recorded as saying, "Thinks Israeli suckered us into this so we can't complain of their sales."

Sen. William S. Cohen (R-Maine) reminded Regan, however, that McFarlane's testimony about the initial phases of the Iran initiative had raised questions about whether the United States had, in fact, been "dragged into this affair."

"Not only was {McFarlane} given authority but the president was pleased with the success of the operation that culminated at least initially with the release of {hostage} Benjamin Weir," Cohen said, noting that access to the president's notes might resolve the apparent discrepancy.Staff writer David Hoffman and researcher Michelle Hall contributed to this report.