IF THE FEDERAL Election Commission were bent on fatally undermining the federal campaign finance laws -- and maybe, we're beginning to think, it is -- it couldn't have done a better job than it did last Thursday in its decision on soft money. Soft money is FEC-ese for contributions that are illegal under federal law but legal under the laws of a particular state, and a 1978 change in the federal law allowed national parties to collect such soft money and disperse it to the state parties where it was legal. This they have done, to a great and unspecified extent. But they are not under any obligation to tell anyone the total amounts they have raised and who has given it.

The practical result is that a fat cat capable of giving $100,000 -- or a corporation or a union, because contributions from them are legain some states -- can give a national party operative that sum of money and not have to disclose it except for much smaller amounts that may have to be disclosed in one or another state. So we're back to the good old days. You can anonymously purchase vast influence, with nobody but the people you want knowing a thing about it. The national parties are now raising millions this way. The Republicans have had the decency to disclose to some reporters the names of contributors and the amounts they have given. The Democrats refuse to do even that.

To all of this the FEC has now just said: fine. It has rejected Common Cause's call for centralized reporting and disclosure. It rejected its staff's much weaker recommendations for codifying existing formulas allocating the money to states and requiring parties to disclose the overall dollar figure of all federal and soft-money contributions. The commission's majority dismissed as "boastful comments" the admissions by Reagan and Mondale managers that they raised $2 million and $6 million respectively in soft money in 1984. But it flatly refused Common Cause's request that it make its own investigation of soft money, although the commission clearly has the power to do so.

The point is that the FEC has turned its back on its duty and has approved the single, most glaring loophole in the campaign finance laws. It has seriously undermined two basic principles that have been at the heart of federal campaign finance laws since 1971: first, that contributions should be disclosed, so that all may judge whether undue influence is acquired; second, that contributions should be limited, so as to limit the influence of the very rich and of those who, as custodians of the funds of corporations and unions, can easily spend large sums of other people's money. The commission's shocking and disgraceful action requires Congress to step into the void and take some action. It may not be able to come up witha perfect solution to the problems posed by soft money, but it could hardly do worse.