A substantial majority of Americans believes that former national security adviser John M. Poindexter is withholding important information in his testimony on the Iran-contra affair and is covering up for President Reagan, while many continue to believe that the president has not told the full truth about his role in the affair, according to a Washington Post-ABC News survey taken after Poindexter's first day of testimony.

But the poll also found what may be a surge in public support for U.S. aid to the Nicaraguan rebels after the testimony of Poindexter and Lt. Col. Oliver L. North, the former National Security Council aide. And while those questioned had doubts about Poindexter's testimony, they expressed strong support for North and said they felt he was telling the truth.

In his first day before the committee, Poindexter said he had not told the president of the diversion of Iran arms sales money to the Nicaraguan rebels. In another part of his testimony, Poindexter confirmed Reagan had signed a presidential finding in December 1985, explicitly authorizing a swap of American arms for hostages. Reagan has said he did not make such a trade of arms for hostages and cannot recall signing the document.

The nationwide telephone survey of 767 people found widespread skepticism of Poindexter's overall testimony. Only one in five people expressed the view that Poindexter is telling all he knows. Two-thirds of those surveyed said Poindexter is "covering up" for others in the administration, and a majority said he is covering up for the president.

On a specific aspect of Poindexter's testimony -- that he did not tell Reagan of the diversion -- those questioned were evenly split, with 48 percent saying he is telling the truth, and 44 percent saying he is not.

The survey also questioned whether the information coming out of the congressional hearings had tended to back up Reagan's statements about his own role in the Iran-contra affair. Of those asked, 48 percent said it did, and 40 percent said it did not -- an improvement for the president from late June, when 39 percent said it backed up Reagan and 52 percent said it did not.

White House spokesman Marlin Fitzwater said the president was "gratified" by Poindexter's testimony that Reagan did not know of the diversion. But the survey showed that in a number of respects the president's credibility remains in doubt.

For example, three-fifths of those questioned said Reagan has not been telling the truth about the Iran affair, a response similar to earlier surveys. The public also remains split over the question of whether Reagan organized an attempt to cover up the facts about the Iran-contra affair.

After nationally televised testimony by North and Poindexter, more than two-thirds of those questioned said the aides were acting under direct orders from someone higher up in the administration. But the poll also found a slight increase in those who said the two men were acting on their own -- a shift in sentiment that may be due to the extensive media coverage of their testimony.

On Poindexter's assertion that Reagan signed the 1985 finding that explicitly authorized an arms-for-hostages trade, the poll showed more than two-thirds of those questioned believe that Reagan did sign the document. Nearly two-thirds also say Reagan is not telling the truth when he says he cannot recall signing the document.

The results suggest a dramatic shift in public opinion on support for the contras. Throughout Reagan's presidency this has been one of his most unpopular initiatives, with a wide majority consistently opposed to funding the contras. But in the latest poll, more than two of five questioned, 43 percent, said they favored granting military aid to the contras. In June, only 29 percent approved the aid. Differences in the way the two polls were conducted make comparisons risky, but the 14-point difference suggests a significant shift in opinion.

Although Reagan and Poindexter both appeared to have credibility problems, those surveyed expressed strong confidence in the credibility of North, who acknowledged shredding documents and lying to Congress in the cause of helping the Nicaraguan rebels.

Asked whether North had generally told the committees the truth, 79 percent said he had, compared to 18 percent who said he lied.

Meanwhile yesterday, Fitzwater said the White House would be interested in working with Congress to change congressional procedures for handling covert operations. Fitzwater said all covert operations had been reviewed earlier this year and a number had been terminated or combined as a result.

"I think we are willing to explore with the Congress this process by which intelligence information is shared with them," Fitzwater said. "There has been a lot of talk during the hearings and in recent days about the number of people who see" the information. He mentioned, for example, the possibility of creating a joint congressional intelligence committee, which Vice President Bush suggested a few months ago.

Fitzwater said White House chief of staff Howard H. Baker Jr. was "simply signaling that we are willing to work with the Congress" when he raised the idea of changes in an interview published yesterday in The New York Times.

"We think the current system is working in a general sense," Fitzwater said.

Washington Post director of polling Richard Morin contributed to this report.