Most Americans would look favorably on a presidential pardon for key figures in the Iran-contra affair, a new Washington Post-ABC News poll shows.

By ratios of 2 to 1 and 3 to 1, they reject criminal prosecution of former national security adviser John M. Poindexter and former White House aide Lt. Col. Oliver L. North. Majorities would approve a formal pardon of the two men by President Reagan to forestall prosecution or punishment.

The poll showed continuing strong skepticism about Reagan's claims -- buttressed by Rear Adm. Poindexter's testimony -- that he was unaware of the diversion of funds from the secret arms sales to support the contra forces opposing the Sandinista government in Nicaragua. But only one-third of those questioned said they would be bothered "a great deal" if Reagan had known.

The response to that and other questions suggested that many Americans see the whole affair as having limited importance. Only 40 percent said they thought Reagan had made "major mistakes" in this matter -- down from 52 percent at the time of the Tower commission report in February.

Meanwhile, North continues to hold the highly favorable ratings he won in his week on the witness stand during the congressional investigation, and his pleading of the case for aid to the contras has increased support for that cause -- although that support shows some slight signs of flattening out below a majority level.

The latest survey showed 41 percent favoring U.S. military aid to the contras and 49 percent opposed -- slightly below the 43 to 46 percent split in a July 15 sampling, but well above the 29 to 67 percent negative response in a June survey preceding North's dramatic testimony on his efforts as a National Security Council aide to channel help from private individuals and foreign governments to the "freedom fighters" in Nicaragua.

The latest reading on public opinion consisted of telepehone interviews with 756 adults conducted Tuesday night, just after Poindexter finished his last day on the witness stand.

The admiral was a far less persuasive witness, the sample said, than North had been. Only 53 percent said they thought Poindexter was generally telling the truth, compared to 80 percent who said they believed North. Sixty percent said they thought Poindexter was covering up for Reagan, and 73 percent said they thought he was withholding important information.

The respondents split evenly on whether they believed Poindexter's crucial statement that he never informed Reagan of the funds' diversion. But by a 63-to-32 percent ratio they said they believed that Reagan knew of the diversion before the Justice Department briefing last November which he says gave him his first intimation of the affair. The The survey suggests that the public puts this whole affair in a different category from the Watergate break-in and cover-up.

skepticism on that point is about the same as that measured before North and Poindexter testified. Half those sampled said they think Reagan -- despite denials all around -- did personally approve the diversion.

Nonetheless, the survey suggests that the public puts this whole affair in a different category from the Watergate break-in and cover-up, which led to impeachment efforts and the resignation of President Richard M. Nixon. The difference is reflected in the attitudes toward pardons and prosecutions.

A Gallup Poll taken just before President Gerald R. Ford pardoned Nixon in 1974 to foreclose any prosecutions showed a 58 percent majority favored putting the former president on trial, and 53 percent said he should not be pardoned if convicted. Two years later, during the 1976 campaign, a 55 percent majority told the Gallup interviewers that Ford had been wrong in pardoning Nixon.

By contrast, in this Post-ABC News survey, 74 percent said North should not be prosecuted for his actions, and 61 percent said Reagan should use his pardon authority to prevent such prosecution. Another 5 percent said that even if North is charged, Reagan should intervene to prevent a trial. For Poindexter, the figures were 63 percent opposed to prosecution, 46 percent for an early pardon and another 8 percent favoring pardon if he is charged.

The White House this week said a presidential pardon is not now under consideration.

The Post survey indicated that North's popularity and credibility translated to greater public support for a presidential pardon in his case than in Poindexter's. It showed 68 percent of the Republicans surveyed and 57 percent of the Democrats favored an early pardon of North. A pardon for North was favored by 64 percent of the women and 57 percent of the men. In all, 756 persons, 18 and older, were interviewed at random. Figures are subject to a theoretical margin of sampling error of plus or minus 4 percentage points. Some of the methods used to complete all interviewing for this poll in one night represent another potential source of error.

Figures are from a Washington Post-ABC News telephone poll of 756 adults Tuesday.