Political fallout spreads from Iran-contra revelation. Reagan's difficulties not seen as comparable to those faced by some former presidents

President Reagan's popularity remains high, but his ability to control the policy agenda in Washington appears to be rapidly waning. Revelations about a secret slush fund for the Nicaraguan contras have compounded Mr. Reagan's problems during the final two years of his term, when a president's power traditionally weakens.

``The sun was setting anyway on the Reagan presidency,'' says political analyst Stephen Hess of the Brookings Institution. ``The question was just how fast the sun went down.''

Unless Reagan can quickly recover, 1987 could see a standoff between the Congress, now under Democratic control, and the White House on a wide range of issues, from trade to defense spending.

Some Democrats, sensing that the President is wounded, are expected to grow bolder in their criticism of Reagan. The Teflon president, they say, has finally been scratched.

Adding to Reagan's problems has been the apparent disorder within the US foreign policy establishment, the sniping among his own staff, and the fallout from Republican loss of the Senate this month.

While Reagan's troubles are great, a number of analysts warn that they should not be exaggerated.

Horace Busby, a veteran of the Johnson administration in the 1960s, discounts some of the parallels being drawn between Reagan's current troubles and those of Nixon (Watergate), Ford (the Nixon pardon), Johnson (the Tet offensive), Kennedy (the Bay of Pigs), and Eisenhower (the U-2 incident).

First of all, this crisis doesn't involve a provocative act against a foreign power, such as those under Eisenhower and Kennedy, according to information now available. Nor does it involve a lie by the President, such as Nixon and Watergate.

Rather this involves a case of abuse of power by people in the White House - an abuse that has been promptly revealed and dealt with by the President.

Certainly, it doesn't help the President, Busby says. Democrats will use the incident to haze Reagan in coming months. But there is no indication that Reagan himself was hurt.

How then can the President regain his momentum? It won't be easy. But Mr. Hess, the author of books on the presidency and Congress, says Reagan could make a comeback under at least two possible scenarios.

A major crisis. Current confusion in Washington could lead a foreign power to challenge US interests somewhere around the globe. Any sort of major crisis would quickly rally the American people to the President.

An arms-control breakthrough. The Soviets, sensing the President has been weakened, might decide they could get their best deal on arms control right now, rather than waiting for a new president in 1989. Reagan would be under pressure to bargain in good faith. The result might be a major arms deal that would take the heat off the the White House.

Historian James MacGregor Burns notes that, despite his weakened position, Reagan still has significant power, including his ability to make war. Dr. Burns says that if Reagan becomes frustrated, he might seek a ``quickie victory, as in Grenada in 1983, with the most natural place being Central America.''

But Hess, who served as an assistant to Eisenhower from 1959 to 1961, rejects that notion. ``Presidents don't create crises to save themselves,'' he says. While the President's current problems crimp his ability to win new programs and launch new initiatives, some experts say that probably is not of great consequence.

Most of Reagan's major policy forays - tax cutting, military buildup, a cap on social spending - have already been achieved. Further, the 100th Congress, under Democratic control, will be dominated by presidential politics. Official Washington is already looking ahead to 1988, the first time in 20 years that there will be an incumbent president not seeking reelection. Partly for that reason, experts expect more political posturing than substance achievement during the next two years.

Under these conditions, Dr. Burns says, Reagan can strive for a few victories, use his ``magic in the media,'' and wrap up his administraton two years hence with the assurance that ``he's home free.''