For Reagan: a full menu of challenges

Everywhere Ronald Reagan looks there's trouble. There's opportunity, too, of course, but now there are accumulating tests on domestic and world affairs. Left behind is the political honeymoon; Mr. Reagan faces the ancient problem of dealing with Congress under separation of powers. And problems hammer on White House doors, be they the MX missile debate or the Cancun conference.

One problem is the domestic economy. For the first time, the President admitted that the country has slipped into a slight recession. ''I think everyone agrees on this,'' Mr. Reagan said as he boarded a helicopter for Williamsburg, Va. The rest of the world, already suffering from recession and inflation, watches anxiously. The Kemp-Roth supply-side economics which were supposed to stimulate industry through tax cuts haven't worked so far, though inflation has declined. More likely, some begin to speculate, is continued unemployment and high interest rates, and the normal course of a cyclical revival some time next year.

Mr. Reagan has plunged into a debate with Congress on the budget. It seems more complicated now than it did when he was a candidate a year ago. Congress originally agreed to budget cuts, tax cuts, defense boosts and relaxation of regulations. But now protests mount over even initial sacrifices, and Congress has stalled. Nearly every President has had the same problem; only in the US among democracies can the legislature defy the executive by increasing expenditures. Addressing visiting editors here Oct. 16, Mr. Reagan advocated a presidential ''item veto,'' charging that the budget process ''doesn't make sense.'' Such power of the purse strings, unlikely to be granted by Congress and probably requiring a constitutional amendment, would shift the balance of power from the legislative to the executive branches.

President Reagan is facing his first all-out test with Congress on a foreign policy issue - the proposed $8.5 billion sale of sophisticated radar (AWACS) planes to Saudi Arabia. The House overwhelmingly voted against the deal, which will go through unless the Senate also disapproves.

Mr. Reagan's foreign policy problem has yet to be outlined in detail. He supports the Senate's failure to ratify the Salt II treaty with the Soviet Union and has taken a bellicose attitude toward the Soviets without formulating details on specific issues. In the meantime there has been a big peace demonstration in West Germany and pacifism continues to grow among some NATO allies. Many signs point to a forthcoming showdown in which the Soviet Union's own problems play a role. The failure of the communist collectivized system to supply adequate consumer goods has brought an extraordinary rift with Poland. In Philadelphia last week, the President taunted the Soviets because they buy food from capitalist America. ''Who's feeding whom?'' he asked.

Mr. Reagan travels to Cancun, Mexico, this week for a two-day meeting with 22 heads of state. The conference is an outgrowth of the 1980 Brandt Commission report which emphasized the global instability that rises from the separation of rich and poor. In the Philadelphia speech, Mr. Reagan promised to keep the American market open to exports from poor countries and urged other rich countries to do the same. He adjured less-developed countries to put their faith less on government assistance and more on private investment and free enterprise.

Overall problems assailing Mr. Reagan put his prestige to a test at home and abroad. He has maintained his personal popularity among the public as a whole. But in foreign affairs, where Mr. Reagan has had little previous experience, his most severe challenges now arise.

Answers to big problems - economic prosperity, nuclear rivalry, and a peace formula - lie ahead. Meanwhile, the customary difficulty of keeping the executive and the legislature in step increases.