In 1968, Congress passed a law to bar racial and other discrimination in the sale or rental of housing. But while Congress was firm in its expression of intent, it was, as often happens, infirm in the means it provided to carry out its intent.

The results were predictable. A widening gap between promise and fulfillment produced frustration, followed by proposals to strengthen the law, followed by lobbying, both pro and con.

But there is something special about the lobbying against the proposed Fair Housing Amendments of 1980, which President Carter has called "the most critical civil rights legislation before Congress in years," and which are set for consideration tomorrow by the House Rules Committee.

In the last few days, about 4,000 local officials around the country have gotten "Dear Mayor" or "Dear County Executive" letters urging them to tell their members of congress to support a crucial, weakening change in the amendments.

The letters were written on official House stationary and were mailed in congressional envelopes. But it was the National Association of Real Estate Boards that reproduced and mailed the letters and paid the postage. The total cost was between $900 and 950, the NARB's Joe Winkelmann, director of legislative liaison, told a reporter.

The signers of the letters were Reps. Ronald M. Mottl (D-Ohio) and John W. Wydler (R-N.Y.). Mottl and Winkelmann agree that the text was a collaborative effort. It was "drafted partially by our office and partially by Mottl's office," the NARB official said in a phone interview. Mottl enlisted Wydler, he said.

The two legislators are co-chairman of the Suburban Caucus, composed of 52 House members whose districts generally have large contingents of suburban voters.

Of the 52, Federal Election Commission records show, 39 have received campaign contributions from the NARB's political action committee for the 1978 and 1980 primaries and general elections. The contributions total more in $105,000, including $88,650 given in 1978. By last March 31 only $11,383 had been given for the 1980 contests, but much larger sums will follow. $1

The contributions have "nothing whatsoever" to do with the letters, Winkelmann said. Mottl and Wydler offered similiar strong disclaimers, saying that they were motivated solely by a desire to defeat what they consider to be bad legislation.

Starting in 1977, the realtors' committee has contributed $1,200 to Mottl, $3,000 to Wydler, who is not seeking reelection, and $200 to Rep. Marty Russo (D-Ill.), secretary-treasurer of the caucus.

By contrast, the committee has contributed $14,000 to caucus member Henry J. Hyde (R-Ill.), sponsor of an unsuccessful proposal to let real estate appraisers allow for racial and religious factors if these are shown to be relevant to the appraisers' calculations of property values. The committee also has given to House members, including blacks, who oppose its stand on the amendments.

Under a House rule adopted in January 1979, members are forbidden to "authorize or otherwise allow a non-House individual, group, or organization to use the words 'Congress of the United States,' 'House of Representatives,' or 'official business,' or any combination of words thereof, on any letterhead or envelope."

The Mottl-Wydler letterhead says, "Congress of the United States / House of Representatives." Each congressman told the reporter that he understood the rule to be directed entirely at use of official stationary to solicit funds or political support for a candidacy, or to solicit money for an organization. The House Committee on Standards of Official Conduct said last Friday that it has received no complaint about the letters.

Wydler told the reporter that the rule was irrelevant in part because the letter had been sent by the caucus.

In fact, he and Mottl each signed the letter as a "Member of Congress." The only mention of the caucus is in the text, which says that the caucus had helped to form it.

Moreover, the Narb's Winkelmann said that the letter was "not sent out on behalf of the caucus." Rather, he said, "it was the action of two members."

The situation originated in enforcement provisions of the Fair Housing Act of 1968 that authorize the Department of Housing and Urban Development only to try to settle disputes between persons aggrieved by purported discrimination -- on account of race, color, religion, national origin or sex -- and alleged violators.

As originally introduced, the new amendments, sponsored mainly by Rep. Don Edwards (D-Calif.), would empower HUD to enforce the law by bringing an administrative action on its own initiative or on behalf of a person who files a complaint.

An administrative law judge would hold a hearing and, if the judge found a violation, could order the violator to stop discriminating and could fine the violator up to $10,000. The decision could be appealed directly to a federal appeals court. The process is similar to that for established agencies such as the National Labor Relations Board.

Last February, the House Judiciary Committee defeated, by a vote of 10 to 20, a proposal that would have cut the heart out of the bill by striking HUD's authority to bring administrative actions.

The proposal -- restoration of which is sought by Mottl, Wydler and the NARB in the letters to local officials -- was sponsored and will be re-introduced on the House by Reps. F. James Sensenbrenner (R-Wis.) and Harold L. Volkmer (D-Mo.). Each has received $3,300 in contributions from the NARB political action committee since 1977.

The House Judiciary Committee adopted by voice vote a compromise proposal under which a ruling by an HUD administrative law judge can be appealed to a U.S. District Court, thus providing another appellate layer between the department and the higher appeals court.

The compromise was agreed to by the Leadership Conference on Civil Rights, a coalition of 150 civil rights, labor, religious, women's and other groups, in order to pick up essential GOP votes, said Joseph L. Rauh Jr., the conference's general counsel. The compromise proposal was sponsored by Reps. Robert McClory and Tom Railsback, both Illinois Republicans. The NARB committee has given $1,500 to Railback since 1977.

Reps. Mottl and Wydler say in their letter to local officials regarding the pending amendments that the HUD administrative tribunal could "handle complaints that local zoning is discriminatory" and that this particular power "violates every principle of sound government because it puts the federal government directly in local affairs."

Rep. Edwards rejected this charge.

"With respect to zoning," the amendments sponsor said, "what is illegal now will continue to be illegal -- no more and no less. It is utter nonsense to say that by putting some teeth into the present law, local decision-making over land use will be preempted."