THE MARITIME LOBBY, with its money bags, has scored a great political triumph. The costs will be borne over coming years by everyone who buys oil products. It's another defeat for the enlightened trade policy that President Carter keeps proclaiming but never quite manages to put into practice.

The maritime interests - ship builders, ship operators and unions together - already enjoy more different kinds of subsidy than any other American industry. Despite this torrent of aid, they have worked their freight rates up so high that nobody uses American-flag shipping unless the law requires it. The maritime lobby has now obtained President Carter's misguided support for its perennial bill to force part of this country's oil imports into those American-flag tankers. The proportion of imports affected will be small at first, but it will rise steadily.

It's called a cargo preference bill, and it is 200-proof protectionism. It would violate American treaties with other nations. It would be inflationary. It would add still another layer of subsidy, this time paid directly by consumers in the form of higher oil prices. As national policy, it's got absolutely nothing going for it but the lobbying muscle and the extensive campaign contributions of the people who wanted it. But that turned out to be enough to get the President's support.

Mr. Carter's whole trade policy is rapidly deteriorating. He keeps talking in terms of the most high-minded commitments to open trade and international competition but, unfortunately, the actual record points the other way. There have been three major cases so far, and three times the administration has caved in to the protectionists.

In the case of the color television sets from Japan, Mr. Carter resorted to an Orderly Marketing Agreement. That term is a euphemism for an agreement by the exporting country to limit shipments. In the shoe case, the administration imposed another couple of Orderly Marketing Agreements on Korea and Taiwan. They will save some jobs in American retail stores as well as increase prices to American consumers. But at least an Orderly Marketing Agreement is the general trade rules that this country has bound itself to. The cargo preference bill is neither.

It's a strange performance for an administration that foresaw trade troubles and staffed itself with people to meet them constructively. Those people, led by Treasury Secretary Michael Blumenthal and Under Secretary of State Richard Cooper, have been walloped in each successive test. The President has been mainly following the advice of Robert S. Strauss, currently his special trade representative and formerly chairman of the Democratic Party. Like most party managers, Mr. Strauss thinks in terms of constituencies and grievances: a factory here, a union there. He apparently does not work in terms of broader concerns, like consumer interests in general, or American productivity, or the future of the highly competitive American industries whose overseas markets are now threatened by foreign retaliation.

If Mr. Carter will not resist protectionist pressure, he cannot expect the fragile government of Western Europe to do better. He may get heat from aggrieved unions - but unlike, for example, the president of France, he does not face an election next year that might bring the Communists to power. In May, at the London summit conference, Mr. Carter joined in a pledge to support firmly the principle of open trade and a growing world economy. The people who heard him must wonder what he had in mind when they now read the cargo preference bill that he had promised to support.