Dense thickets of baffling problems stretch before the American people for the rest of this century. A hope was that the presidential campaign would illuminate political solutions. But the Billy affair assures that much of the campaign will be dominated by a scandal peripheral to the country's serious business.

At the core of the true trouble lies slow economic growth. A declining national surplus makes it increasingly hard to accommodate demands for higher consumption, more welfare, a better quality of life and stronger defenses.

A multitude of reasons enters into the slowing of growth. Persistent inflation stimulates recessions and curtails recoveries. Energy shortages fuel inflation. A small propensity to save on the part of the "now" generation makes less money available for investment innovation. New entrants into the work force reduce output per worker. Self-protective arrangements by business, labor, minorities and various other groups strain the welfare state and reduce the capacity of the private sector to respond to challenges. All these phenomena make for a weaker American position abroad.

No sure line of escape has yet been traced. But a start lies in public awareness of the trouble. Otherwise there can be no consensus for the sacrifices required for a safe transit of the danger zone ahead.

The presidential campaign held out a possibility for deeper understanding. The Republican strategy has been to make President Carter the issue. In the process, it might have become clear how inadequate mere moralism was as a guide to public policy.

The Carter strategy has been to make Ronald Reagan the issue. In the process, the perils of oversimplification might have emerged.

The Billy affair, however, guarantees an orgy of investigations by Congress and the press into the wrongdoings of the Libyan government, the swindler Robert Vesco and the president's goofy brother. Any immediate Democratic hope of making Reagan the issue is now shattered. Oversimplification will not go on trial until at least the first presidential debate in September.

As to Carter's moralism, it is apt to emerge reinforced. There is no smell of Watergate about the present mess. All signs suggest the fix was not put in -- not for Libya, nor for Vesco, nor for Billy. If anything, the administration will probably look good on what the president's counsel, Lloyd Cutler, calls "impropriety on the law enforcement side."

Indeed, far from being an instance of wrongdoing, the whole sad business provides new evidence of the harm done by sanctimonious piety. In keeping with the pretense of virginal purity, the Carter administration arranged a kind of divorce between the White House and the Justice Department.

The department did not keep the president abreast of its investigation of his brother in any systematic way. When Billy stepped into dangerous waters, Jimmy was under no pressure to bring to bear the kinds of discipline Lyndon Johnson and Richard Nixon applied to their erring siblings. So Billy carried his clownish greed into an embarrassing affair of state.

Further savor was imparted to the affair by the meetings that an obscure Libyan diplomat, Ali Houderi, had first with Zbigniew Brzezinski, the president's national security adviser, and then with the president himself. The White House says the meetings were arranged with an eye toward freeing the hostages in Iran. But how did the White House reach the conclusion that the Libyans -- bad actors in general and responsible in particular for kidnapping a Lebanese religious leader close to the Ayatollah Khomeini -- had enough clout in Tehran to warrant the personal intervention of the president?

The answer lies in a romantic illusion entertained by the Carter administration about its rapport with countries of the Third World. The off-the-wall intervention by Mrs. Carter in behalf of the Libyan connection is one example. Brzezinski provides another. He behaved irresponsibly at the Great Wall in China, at the Khyber Pass in Pakistan, at the 25th anniversary of the Algerian revolution and in dealing with Libya. In each case, he imputed to those countries a disposition to help the United States, which they do not begin to have.

Why? Because much as Andrew Young saw in countries struggling to be independent the equivalent of the civil rights movement, so Brzezinski read into Third World nations the anti-Soviet and pro-American character of his native Poland. Thus instead of protecting the president, his adviser egged him on to Third World follies.

In the present instance, the eclipse of the real issues causes all of us to suffer. The only justice is that the biggest loser is the man whose unique, patented brand of innocence has brought on this latest bout of circus politics. p