WASHINGTON -- President Clinton said last night that he would release copies today of his income tax returns for 1977-1979, years in which he and Hillary Rodham Clinton first invested in the Arkansas real estate deal known as Whitewater.

Mr. Clinton also revealed something his critics have maintained for months -- that he and his wife lost less money in the Whitewater partnership than they have claimed for more than two years.

Only hours before the president's news conference -- his latest attempt to stop the political bleeding over Whitewater -- a prominent Republican critic, Rep. Jim Leach of Iowa, claimed that the Clintons might actually have profited from the investment.

But the president said he was "certain" they had lost money on the deal.

Mr. Clinton conceded that, in light of the damage being caused by the Whitewater affair, he and his staff "weren't as sensitive as we should have been" to the appearance of White House interference in the investigation of a failed Arkansas savings and loan linked to the Whitewater Development Corp.

"But . . . the one thing you have to say is, you learn things as you go along in this business," he added.

Mr. Clinton continued to maintain that he and his wife have done the same thing that millions of middle-class Americans have done -- lost money in a business investment. He insisted that, as president, he is cooperating to an unprecedented degree with Robert B. Fiske Jr., the special prosecutor investigating the matter.

"Cooperation, disclosure, and doing the public's business are the order of the day," he said.

The defense was a familiar one, but the setting was not: an evening news conference in the East Room of the White House, only the second of his presidency.

The session was timed to reach the largest home viewing audience and confront a national press corps that has become )) increasingly consumed with the Whitewater story. It also was designed to bolster Mr. Clinton's sagging popularity, as new polls confirmed that Whitewater has eroded public confidence.

Republican Rep. Newt Gingrich of Georgia praised Mr. Clinton's "sheer technical skill" at framing the issue "on his terms."

But he went on to say, "I think on any given day I am always impressed with how believable he is, and then the following day I start reading the newspapers again."

Mr. Clinton was on the defensive for most of the 38-minute, televised session, which was dominated by questions about Whitewater.

In an apparent shift of strategy, Mr. Clinton shrugged off an opportunity to blame Republicans for the furor surrounding Whitewater. He said, mildly, that it is not "useful to get into blame. . . . I think that the worst thing that can happen is for me to sort of labor over who should be blamed [for] this. There will probably be enough blame to go around."

Mr. Clinton refused to give in to critics who say that the real "crime" at the root of Whitewater is that he benefited from the "greed decade" of the 1980s, the period of Republican rule that he repeatedly condemned during his presidential campaign.

"The fact that we made investments, some of which we lost money on, some of which we made money on, has nothing to do whatever with the indictment that I made about the excesses of the '80s," he said.

He strongly defended his wife.

In the evening's most animated exchange, Mr. Clinton said emphatically, "Absolutely not," when asked if her moral authority has been eroded and whether he was reconsidering her lead role in health care reform.

"People should not be able to raise questions and erode $H people's moral authority in this country," he said. "There ought to have to be evidence and proof."

The question-and-answer session came only a few hours after Mr. Leach, in a lengthy speech on the House floor, said Whitewater was "about the arrogance of power" and contended that the Clintons, who claim they lost $69,000 on their investment, actually profited from it.

Last night, Mr. Clinton said that figure should be about $47,000. He said that some $22,200 in payments that he had previously thought were related to Whitewater actually involved a short-term loan to his mother for a cabin.

Mr. Leach also repeated allegations that the troubled Madison Guaranty Savings and Loan, owned by the Clintons' partner in the land deal, James McDougal, pumped money from the S&L; into the Whitewater partnership.

Madison Guaranty eventually failed, at a cost to taxpayers of up to $60 million.

For more than two years, the Clintons have described their Whitewater investment as a bad deal and said they lost money on it.

But recently, as their lawyers re-examined records, the Clintons have hinted that they may owe additional taxes on the investment, an admission Democratic strategists fear could prove politically damaging.

Mr. Clinton asserted last night that he did not believe he owes additional taxes but would pay more if it turns out that he underpaid.

The Clintons have admitted underpaying their federal taxes for 1984 and 1985, because of improper deductions related to Whitewater. They voluntarily repaid about $4,000 last year.

Their refusal to release their returns for the years 1977-1979, however, had made it impossible to come up with an independent assessment of the Whitewater investment and any additional taxes they may owe.

Mr. Clinton said his attorney, David Kendall, would brief reporters on the documents at the White House today.

In contrast to previous sessions when he's been questioned about Whitewater, the president appeared unruffled. Mr. Clinton did try to draw the distinction between Whitewater and what he views as truly important issues facing the nation.

He opened the news conference by reading a statement that he had described earlier as a report to the American people on the status of his domestic agenda in Congress.

He touched on events such as Wednesday's political assassination in Mexico, the aircraft accident at Pope Air Force Base and the withdrawal of the last U.S. troops from Somalia.

The president provided a status report on the legislation Americans care most about -- the crime bill -- and the measure his administration cares most about -- health reform.

"I know that many people around America must believe that Washington is overwhelmingly preoccupied with the Whitewater matter," the president said. "But our administration is preoccupied with the business we were sent here to do for the American people."

Mr. Clinton acknowledged, however, that the two are linked. Last night's news conference coincided with the release of new lTC national polls that indicate Whitewater is damaging the president politically.

A survey by the Times Mirror Center for the People and the Press, found that a majority of Americans now believe that the Clintons are guilty of wrongdoing on Whitewater.

Two out of three of those questioned (67 percent) believe the Clintons did something wrong, compared with 45 percent in January, although most believe that the offenses were minor.

According to the poll, conducted last weekend, there is almost an even split in the way Americans view Mr. Clinton's job performance, with 45 percent approving of the way he is handling his job as president and 42 percent disapproving.

By comparison, 51 percent approved and 35 percent disapproved in the same poll in January.

A similar ABC News poll last night found that Mr. Clinton's job approval had fallen below 50 percent for the first time this year.

The poll by Times Mirror, which publishes The Sun and other newspapers, linked Mr. Clinton's sagging popularity to Whitewater, with 81 percent of those questioned saying that the affair is disrupting the government's ability to deal with crime,

health care reform and the economy.