WITHOUT fully realizing what it was creating, Congress passed legislation last year which not only forces taxpayers to subsidize the Republican and Democratic parties, but also makes us indirectly finance the American Communist Party, the Nazis, the John Birch Society and a conglomeration of other political groups of every persuasion.

Congress apparently never intended for its new law to help all those organizations. Introduced by Rep. Mendel Davis (D-S.C.), the legislation was supposed to help only the nation's two largest political parties by permitting them to use a cheap, third-class postage rate.

But Davis' legislation was hustled through Congress so quickly that is author didn't have time to polish up its final language. The result: Nearly any group calling itself a political party, from "hippies, yippies and commies to two guys selling gum in New York, all can use the sepcial, taxpayer-subsidized postage rate," explains Van Seagraves, a spokesman for the U.S. Postal Service.

Davis' law approved during the frantic final days of the 95th Congress. It was the type of amendment that attracted Republicans and Democrats alike because it promised them a chance to cut soaring campaign costs and increase political contributions.

And it seemed so simple. All Congress really wanted was the same privilege that every Bible-thumping preacher on a Sunday morning television show noe enjoys.

Davis' amendment said specific national and state Republican and Democratic party committees should be permitted to use non-profit, third-class mail rates.

It costs 8.4 cents to mail a letter by third-class mail-the service used for most fund-raising letters. Under Davis' amendment, the two parties would have to pay only the non-profit rate of 2.7 cents per letter.

There was a catch. The 5.7 cents per letter difference between the two rates would have to be paid by Congress. And that meant that the taxpayers actually would be helping finance every mass mailing by the two parties.

Since some taxpayers might resent such a subsidy, Davis' amandement had to be handled discreetly.

It was Davis introduced his amendment in the House Administration Committee, rather than in Post Office and Civil Service, which usually handles postage rates.

His amendment was added to several others which were being tacked on to the Overseas Citizens Voting Rights Act of 1975. The House voted 327 to 78 to approve the amended act, and the Senate accepted the changes by voice vote without debate. Only and the Postal Service was ordered to comply did Congress discover that it had passed more than it had intended.

"We have no choice"

"The new law is quite clear," says Jim Finch, an assistant postmaster general. "We have no choice. It says 'qualified political parties' shall be able to use the special rate. Our attorneys believe that means any political parties and we figure this new law is going to cost the public about $18 million in 1979-and that's a conservation estimate."

Bob Moss, an attorney for the House Administration Committee, disagrees. He claims the Postal Service dislikes the new law and is trying to undermine it by making it so expensive that Congress will revoke it. Moss says a committee investigation shows the new law only will cost $2.8 million-if it is applied as intended.

"We never intended to help the Communist Party," says Moss. "We thought the Senate would send the act back to a joint compromise committee, but it didn't. The Senate accepted the House version as amended and that meant we couldn't polish up the language. But I believe the law still is ambiguous enough that the post office could issue its own regulations and forbid the use of the special rate by anyone except the Republican and Democratic party committees which we intended the amendment to help."

The Postal Service's Finch disagrees.

"Look," he says, "We can't walk into a courtroom and tell the judge, 'Your honor, we know that this law says one thing, but Congress really meant it to say this .' That's ridiculous. If Congress wants to clarify it, it can-by passing a resolution."

Moss agrees that a special resolution would clarify the law. But, as one postal official puts it, "How many congressmen would want to publicly support a resolution that guarantees the Republicans and Democrats special treatment at taxpayers' express?"

Rather than chance such a vote, the House committee reportedly is sending a letter to Postmaster General Benjamin F. Bailar asking him to draft tougher restrictions to limit the special rate.

"It's hilarious," says Merritt Felice, an attorney for the U.S. Labor Party. "Here they were trying to insure themselves special treatment and now they discover that we all are going to get to use it."

The National Socialist White Peoples Party-formerly the American Nazi Party - will use it, says Martin Kerr, a Nazi spokesman. The American Communist Party and John Birch Society also are investigating the taxpayer-subsidized rate.

"Every year Congress attempts to shut out every party but the Democrats and Republicans," Felice charges. "But this time it backfired and we plan to use it [the mailing rate] as much as possible. After all, it's the law now.