President Reagan, acknowledging that "there's nothing I can say that will make the situation right," last night accepted full responsibility for the Iran-contra affair and said aides should not have shielded him from responsibility for making a decision on whether proceeds from the arms sales to Iran were being diverted to aid the Nicaraguan contras.

In a nationally televised speech from the Oval Office that was sober and at times contrite in tone, the president tried to close a chapter on what he called "the Iran-contra mess" and move on to other issues.

"There are now 17 months left in this administration, and I want them to be prosperous, productive ones for the American people," Reagan said.

The president stopped short of directly apologizing for what had happened, but criticized himself for failing to heed the warnings of Secretary of State George P. Shultz and Defense Secretary Caspar W. Weinberger, both of whom had counseled against trading U.S. arms for Americans held hostage in Lebanon. Reagan said he had been "stubborn in my pursuit of a policy that went astray."

While repeating his contention that he had known nothing about the diversion of arms profits to the contras, Reagan pointedly faulted his former national security adviser, Rear Adm. John M. Poindexter, who testified before congressional investigating committees on July 15 that he had decided on his own not to inform the president and that "the buck stops here with me."

"The buck does not stop with Adm. Poindexter, as he stated in his testimony," Reagan said, "it stops with me. I am the one ultimately accountable to the American people. The admiral testified that he wanted to protect me; yet no president should ever be protected from the truth. No operation is so secret that it must be kept from the commander in chief. I had the right, the obligation, to make my own decision."

Reagan did not say what his decision would have been, but aides have said that he would not have approved the diversion.

Reagan recalled the warnings of Shultz and Weinberger "that the American people would immediately assume this whole plan was an arms-for-hostages deal and nothing more. Unfortunately, their predictions were right . . . . I let my preoccupation with the hostages intrude into areas where it didn't belong. The image -- the reality -- of Americans in chains, deprived of their freedom and families so far from home, burdened my thoughts. This was a mistake.

"My fellow Americans, I have thought long and often about how to explain to you what I intended to accomplish, but I respect you too much to make excuses," Reagan continued. "The fact of the matter is that there's nothing I can say that will make the situation right."

Reagan went on to claim, as he did after the Tower board report on the Iran-contra affair appeared last February, that he had "tried to take steps so that what we've been through can't happen again," including changes in his national security and White House team and "new tighter procedures" for informing Congress of covert activities. He said hoped that the executive and legislative branches would emerge from the experience with greater trust in each other and that "this may be the eventual blessing in disguise to come out of the Iran-contra mess."

The president, midway through his 15-minute speech, then turned to a discussion of his priorities for the remaining months of his term, beginning with a pledge to battle for Senate confirmation of his controversial Supreme Court nominee, Appeals Court Judge Robert H. Bork. Hailing Bork as an apostle of "judicial restraint" and an "important intellectual addition to the court," Reagan said, "I will fight for him because I believe in what he stands for."

Democratic opponents of Bork do not dispute his legal or intellectual credentials, but have depicted him as a conservative activist who would tip the balance on the court.

Reagan said his other principal priorities included an arms control treaty with the Soviet Union, a negotiated end to the conflict in Central America and a constitutional amendment that would require a balanced budget.

The president said he hopes to reach "a comprehensive and verifiable agreement" with the Soviet Union on a treaty eliminating intermediate-range and short-range nuclear missiles in Europe and Asia. U.S. officials have said that if such an agreement is reached it would probably be signed at a summit between Reagan and Soviet leader Mikhail Gorbachev late this fall.

"I am optimistic we'll soon witness a first in world history -- the sight of two countries actually destroying nuclear weapons in their arsenals," Reagan said. "Imagine where this might lead."

Reagan said the United States sought "peace and stability" in its relationship with the Soviets and had similar goals in the Middle East. "Bringing stability to this troubled region remains one of the most important goals of my presidency," Reagan said.

The president also said that the "cause of democracy" in Central America will "occupy my time and my heart" during the remainder of his second term. But he stopped short of endorsing a Nicaraguan peace plan approved by five Central American countries, including Nicaragua, in Guatemala last Friday.

Reagan also sidestepped the issue of what will become of the contras fighting the leftist Sandinista government in Nicaragua under this plan. The president said he was "totally committed to the democratic resistance -- the freedom fighters -- and their pursuit of democracy in Nicaragua" without commenting on what their fate would be if the peace plan were implemented.

The president has been under liberal fire for failing to give wholehearted backing to the Central American peace plan; he has also been criticized by conservatives who believe that the peace proposal as well as Reagan's own plan for ending the Nicaraguan conflict would be tantamount to abandoning the contras.

Referring to support for his plan by House Speaker Jim Wright (D-Tex.) and other moderate Democrats, Reagan said he is "especially pleased that in the United States' diplomatic initiative we once again have the beginnings, however uncertain, of a bipartisan foreign policy . . . . I hope this cautious start will grow and blossom."

The president renewed his call for a constitutional amendment requiring a balanced budget, an issue for which he has campaigned repeatedly in public speeches but on which he appears to have made little progress in Congress.

Reagan said "to get things moving" he was proposing that "if Congress agrees to schedule an up or down vote this year on our balanced-budget amendment, then I will agree to negotiate on every spending item in the budget."

Sources said this was an offer by Reagan to negotiate seriously on the defense budget, which Congress has cut, and did not mean that the president is open to a compromise that would require new taxes.

The president warned that if Congress refuses to approve a balanced-budget amendment, the call for a constitutional convention to pass such an amendment "will grow louder." Pointing out that the call for such a convention is only two states shy of being approved, Reagan said, "one way or another, the will of the people always prevails."

But it is unclear, even to White House officials, if Reagan retains the political strength necessary to force the balanced-budget issue with Congress or to prevail on other domestic priorities, including the confirmation of Bork.

Recent public opinion surveys show slight gains in Reagan's approval ratings but find that a majority of Americans, at least until last night, continue to disbelieve his explanation of the Iran-contra affair. A majority of Americans in most polls say they continue to trust Reagan but also believe that his leadership has been damaged by the scandal.

White House spokesman Marlin Fitzwater said before the speech that Reagan had spent considerable time on the Iran-contra portions of the address. The speech, described by one official as "the president's best effort" on the scandal, ignored many of the detailed questions raised by testimony in the hearings and did not deal with what he would have done if he had known about the diversion of funds.

Poindexter asserted that Reagan would have approved the diversion of funds. Reagan has said through spokesmen that he would not have approved of it.

Reagan is to leave today for a 25-day vacation in California, stopping en route to make two speeches in Nebraska that officials said will focus on his priorities for the remainder of his term.

Staff researcher Michelle Hall contributed to this report.