Iran crisis tests Reagan leadership. Reagan is being urged by Republicans as well as Democrats to act decisively to restore trust in his presidency on the Iran-contra affair. His actions could make a difference for the GOP in who runs for the White House in 1988 - and who wins.

This is Ronald Reagan's hour of decision. As the crisis over the Iranian and Nicaraguan arms operations deepens, the President faces the imperative of restoring his credibility and showing that he - not his aides - is in charge. If he acts quickly and decisively, at the least changing the guard at the White House, he may still be able to put the present crisis behind him.

But if Mr. Reagan decides to ride out the storm without some bold action, say political friends as well as foes, he is likely to find himself in increasingly hot water. Even if it emerges that he did not personally know the details of the Iranian and Nicaraguan capers, he will be held responsible for not ensuring good management in his own shop.

Calls for a shake-up at the White House, appointment of an independent counsel to take over the Justice Department investigation of the Iranian arms sales, and a special session of Congress to initiate a Watergate-style probe continue to be heard among GOP lawmakers. It is felt that Congress has to be satisfied that officials at the White House are trustworthy and operating above board.

``I don't think [White House chief of staff] Don Regan can stay even if he's innocent of any knowledge of this affair,'' says a former key Reagan campaign official. ``To get through this the President needs a more able, talented, unencumbered chief of staff. ... Regan has lost his credibility with the people.''

The President stated yesterday that he wants ``all the facts to come out'' and would welcome a special prosecutor if it appeared federal laws were violated. Addressing a special three-man commission set up to probe how the National Security Council (NSC) staff functions, Reagan said he has ordered the staff not to participate in national-security operations until the panel issues its findings.

Although political allies expect the President to decide on Regan's removal, at this writing the President gives no indication that he has reconsidered his policies over the past 18 months. In a Time magazine interview published this week the President calls Lt. Col. Oliver L. North, who was fired from the NSC staff for secretly transferring Iranian arms-sale proceeds to Nicaraguan rebels, a ``national hero.'' ``My only criticism is that I wasn't told everything,'' he states.

Reagan also criticizes the press for ``great irresponsibility'' and refers to the present turmoil as ``a Beltway bloodletting,'' meaning that only people in Washington are roiled by the unfolding saga of US operations in Iran and Nicaragua. ``Frankly, I believe that as the truth comes out, people will see what we were trying to do was right,'' Reagan says. ``I'm not going to back off, I'm not going to crawl in a hole. I'm going to go forward.''

In the opinion of presidential analysts, the current crisis sharply delineates some of the weaknesses of the President's leadership, which have tended to be submerged because of his popularity and the perceived success of his presidency. Reagan's ability to delegate responsibility is generally viewed as a strength, for instance.

But Reagan's lack of knowledge about many issues, his lack of interest in detail, and his heavy reliance on his staff risk opening the door to poor advice and resulting poor judgment.

``Everyone knows he's not a hands-on manager,'' a White House insider says. ``He depends on the advice he gets, and that's why he must get more than one point of view.''

The President especially needs good advisers because he makes decisions largely on the basis of instincts, hard-held ideological notions about issues like communism and the economy, and a strong sense of patriotism and personal loyalty to individuals. It apparently was his preoccupation with the American hostages in Lebanon that led him into the complicated scheme involving covert arms shipments to Iran. He seemed not to be aware of the long-term strategic implications of the Iranian operation or to be troubled about the loss of perhaps many lives because of military help given to Iran.

``He has certain ideas about public policy and doesn't like to be challenged by facts,'' says a leading presidential scholar. ``He also pays no attention to execution.''

It is also not clear that the deepening crisis will have an appreciable impact on the President's remaining years in office, inasmuch as the administration already is running out of steam.

There is little on the White House's domestic agenda for the remaining two years, and the one foreign-policy area that shows promise - arms control - appears to be deadlocked.

``In the last two years of an eight-year period an administration runs out of zip,'' says political scientist James Sundquist. ``They stop bringing in new people, the crowd thins out, and the team gets defensive. It's par for the course. ... Even if the President gets over this, not much can be done.''

Some would disagree. Political allies suggest that, if Reagan moves vigorously by shaking up his staff, appointing respected advisers, and setting a new sense of direction, he will still be able to move forward on the most crucial issue of his presidency: the superpower arms balance.