President's public approval rating falls. Pollsters say Reagan's strength - effective leadership and personal trust - is questioned after Iran-contra revelations

President Reagan is in trouble not only inside the Washington Beltway. He has lost standing with the American people - the source of his political strength. Unless he restores order and credibility to his administration, say leading public opinion experts, he risks creating the impression of another failed presidency.

A New York Times/CBS News poll finds that in one month his public-approval rating has dropped a sharp 21 points to 46 percent as a result of the disclosures about US arms operations in Iran and Nicaragua. According to the poll, a majority of Americans believe the administration is covering up the facts of the arms deals and that the operation is as serious as the Watergate affair which brought down Richard Nixon.

Andrew Kohut, president of the Gallup Organization, says he was surprised by this dramatic decline. ``That Reagan's rating has dropped below 50 percent is extraordinary,'' he says. ``He's got a real problem because even in the early part of his administration when he was not well received, the poll ratings were slow to change.''

Republican polls since the Iran crisis also show a similar dramatic downward slide. Everett Carl Ladd, executive director of the Roper Center for Public Opinion Research at the University of Connecticut, observes that the President's ratings had slipped slightly - to roughly 60 percent - even at the time of the recent election, well before the Iran-Nicaraguan crisis. Then, it slipped further but not sharply (to about 57 percent) after the news of the secret McFarlane mission to Iran. It was only after the press conference by Attorney General Edwin Meese III, announcing that funds from the Iran arms sales had been diverted to the Nicaraguan contras, that polls showed a significant drop.

``So rather than 21 points you have a 10- to 12-point drop in a little more than a week,'' says Dr. Ladd. What is not known from current polling data, says Ladd, is how serious or superficial is the change in public attitude, i.e. the intensity of the change, or the duration of it. The latter will naturally depend on how the public responds to the President's latest steps to gain control of the situation.

The President's lowest point ever was in December, 1982, when deep recession drove down his approval rating to 42 percent.

Reagan's political strength in the past six years has rested largely on a public perception of effective leadership and personal trust. These are now called in question by the disclosures of the secret arms operations.

``Both of these factors have been challenged by this event,'' says opinion analyst William Schneider of the American Enterprise Institute. ``Reagan's effectiveness depends on his personal standing with the public and that has been weakened.''

According to the Times/CBS poll and other opinion analyses, most Americans do not believe that the President was unaware that money from the Iranian arms sales was being transferred to the contras, in possible violation of the law. The vast majority do not believe that White House Chief of Staff Donald T. Regan did not know of the operation.

In general the negative ratings are due not only to the Iranian-Nicaraguan affair but trace also to other instances of ineptness in foreign policy. These include the President's handling of the summit meeting with Soviet leader Mikhail Gorbachev in Reykjavik, the ``swap'' of American reporter Nicholas Daniloff for a Soviet spy, and the Hasenfus affair in Nicaragua. The latest crisis has simply added to a growing public perception of ineptness and loss of control at the White House.

What White House and National Security Council aides did not calculate when they began the covert talks with so-called ``moderates'' in Iran is the antipathy which Americans have for the Ayatollah Khomeini and the consequent political impact of the McFarlane mission on the President's standing.

``Most polls show that Iran ranks lowest in the public's view of foreign countries - even lower than the Soviet Union,'' says Mr. Schneider.

``Iran is not like an opening to China,'' concurs Mr. Kohut. ``Iran is regarded as an outlaw nation.''

Public-opinion analysts agree that Reagan can make a comeback in public opinion if he can get the affair behind him. There remains, however, a store of affection and good will for the President, say experts, and Americans tend to be forgiving of mistakes.

``He can bounce back,'' says Schneider, ``but this is more serious than the '82 slide because it challenges his credibility, whereas in '82 it was the recession.''