As the American people go back to work, the Carter administration is finally coming down to earth. Its claims of moral superiority have been mocked by events, and not just the Lance affair.

Works, not grace, have emerged as the true test of its quality. Which is a good thing for the country and - if it can learn to argue from the record instead of from purity of intent - for the administration.

A certain stress on righteousness was probably inevitable. Carter came on the national scene as an outsider firm in his faith, his family ties, his rural roots and his disdain of Washington insiders. During the campaign he grabbed all the white-hat issues: human rights, open government, reducing waste and corruption, and barring nuclear proliferation. As President he reinforced the boy-scout image by such symbolic events as the walk down Pennsylvania Avenue, the town meeting and a plain style at the White House.

But American life is too complex, the country's international responsibilities too great, for mere good-guy government. Very early souring relations with Russia forced the administration to retreat from emphasis on open diplomacy and human rights, while difficulties with the allies led to a softening on limiting nuclear power in the interest politics with a vengeance to soften the impact of tight job markets and lower prices on blacks and farmers respectively.

The Lance affair comes in the midst of this general process of normalization. If nothing else, Lance has been shown to be a complusive crapshooter, prepared to seize every occasion for self-enrichment. Far from being offended by blatant acquisitiveness, the President and his friends have tried to tough it out by the familiar device of dumping on the news media and the Eastern Establishment. With a crony in trouble, the knight in shining armor has sounded much like any other President - not excluding Nixon.

So like most other Presidents, Carter is now going to have to get by on the strength of performance rather than promises. At first blush that should not be too hard.

Under the Carter administration the economy has done far better in both growth and inflation than was generally predicted last January. A broad legislative program including comprehensive proposals on energy, welfare, illegal aliens and government reorganization has already been surfaced, and a tax plan is due this month. If there is trouble with Congress, it arises from an overcrowded agenda.

In foreign policy, secret diplomacy has put relations with Russia back on the track toward an arms-control agreement. Arab folly has saved the President from the consequences of his first initiative in the Middle East, and a new beginning is possible.

Relations with the allies have weathered the first bout of consciousness-raising on nuclear proliferation. A precipitate withdrawal of troops from Korea has been headed off. A dialogue with China has been opened that does not immediately compromise Taiwan. The administration has achieved in Panama a treaty that enhances the security of American interests throughout Latin America.

The great problem for the administration is that little of this achievement has been recognized. The administration's undoubted performance has been upstaged by its unredeemed promises. A bushel of accomplishments in this world has been hidden by the blinding light of other-worldly claims.

So the Carter administration now faces a challenge in communication that no recent administration has handled effectively. It is the challenge of delivering to a hyped-up public the message of modest success in dealing with intrinsically hard problems.