THE MARITIME LOBBY, with the help of its many friends in the White House and Congress, has invented a different kind of revenue sharing. The system has a certain scientific interest, since it's beginning to look like the political equivalent of the perpetual-motion machine. The maritime lobby - the unions, the ship operators and the shipyards - invests wisely in certain elections. The politicians express their gratitude by extending and augmenting the enormous subsidies that go to the U.S. merchant marine. That further enriches the maritime industry, enabling it to pour still larger contributions into political campaigns the next time around. It's a delightful system for everybody except, of course, the taxpayers and consumers who supply the endless subsidies that keep the wheel suring.

To see the system busily at work, look at the cargo-preference bill. It would require a rising proportion of American oil imports to travel in ships built and registered in the United States - at freight rates several times as high as any other shipping costs. The lobby wanted that bill in the last Congress, but could not get it past President Ford. Last year, the maritime interests invested $100,000 in President Carter's primary campaign. That foresight paid off, and they now enjoy the President's endorsement.

Congressional enthusiasm for the idea is unlimited. In June the chairman of the House Merchant Marine Committee, Rep. John M. Murphy (D-N.Y.), held a fund-raising party at the Mayflower Hotel. The maritime lobby turned up in force and, as Common Cause reported, donated $9,950 to Mr. Murphy's future. It was an odd time to hold a fund-raising party, since there is no election this year. But it came in the midst of the hearings that Mr. Murphy was holding on the cargo-preference bill.

When the bill came ot a vote a month ago, the committee reported it by a heart-warming majority of 31 to 5. Common Cause, which follows the maritime lobby with the fascination of a pathologist tracking the plague baccillus, observed that the maritime unions alone spent $83,263 for campaign contributions last year to 26 members of the committee. All but two of them voted for the cargo-preference bill.

All combined, the lobby spent something over $1.1 million last year on contributions to Mr. Carter and selected congressional candidates. That might sound like a lot, but it's a modest sum compared with the returns that will flow in when the bill is passed. The Government Accounting Office says that it will cost consumers at least $240 million a year in unnecessary freight charges - a figure, incidentally, more than twice as high as the dubious estimate to which the administration testified. Costs could go much higher, the Secretary of the Treasury warned Mr. Carter in a private memo that has leaked - and he added that the bill is inflationary, would create unemployment, would invite retaliation and would violate American treaties with more than 30 countries.

That Treasury memo is right on all counts. But public policy won't have much to do with this bill's steady progress toward passage. The maritime lobby takes the straightforward view that it bought the bill at the going price, and it is entitled to prompt delivery.