An agreement between the Reagan administration and the politically powerful National Association of Realtors gutted proposed legislation designed to strengthen enforcement of laws against housing discrimination, several witnesses told a Senate subcommittee this week.

Sen. William Proxmire (D-Wis.) said the hearings were called because the Department of Housing and Urban Development made a deal with the NAR, long an opponent of some anti-discrimination enforcement methods, without consulting any other private organizations. He said fair housing groups were "outraged" that HUD had "not once negotiated" with them before reaching the agreement, which was then incorporated into anti-discrimination provisions of housing authorization bills sent to Congress.

Galen Martin, executive director of the Kentucky Commission on Human Rights, told the Senate housing and urban affairs subcommittee, "HUD has no business making deals with the Realtors. HUD ought to be on the side of law enforcement."

The NAACP has "grave reservations" about the agreement and the "undue influence of one group," Althea T. L. Simmons, director of the NAACP's Washington bureau, testified.

HUD Secretary Samuel R. Pierce Jr. decided to negotiate with the NAR because the organization's opposition to the original proposal for funding private testing was "a source of concern to us," HUD general counsel John Knapp told the subcommittee. Officials felt "the success of the program would be in some danger" if enacted over the opposition of the powerful organization, he said.

William D. North, executive vice president of the Realtors' association, said the agreement reached by his group and HUD "would solve many of the concerns and abuses" that have so often brought the real estate industry and fair housing groups "to loggerheads." He said discrimination cases are too easy to file and too difficult for real estate organizations and individual agents to fight. He accused housing organizations of filing lawsuits as a way to raise money for operating expenses.

"There is a very serious invasion of personal rights and privacy in testing where there is no underlying complaint," North said.

The legislation now awaiting consideration by the Senate contains all the changes resulting from the controversial agreement, but the House cut out several of the provisions before passing its housing bill last week.

HUD's agreement with the 700,000-member NAR altered key sections of a $3 million proposal to fund private fair housing groups that test communities to determine whether real estate sales organizations and landlords are discriminating.

Testing, which private organizations have used for several years, is conducted by people posing as potential buyers or renters who try to rent or buy housing denied to black or other minority home seekers. Testers are sent out when allegations of bias are received against specific landlords or real estate agents, and to communities where the fair housing organizations believe discrimination is practiced, although no complaints have been lodged. If the white testers are told they may rent or buy the same property, the testimony of the testers is used as evidence of discrimination.

Under the agreement with the NAR, testing could be conducted only where complaints are received, and more testers would be required than is now the practice. When bias complaints are made against sales organizations, the two testers with closely matching income and other characteristics except for race must find the individual agent who first talked with the home seekers who filed the complaints and again attempt to buy a house through that agent. In other changes to the original legislative proposal, restrictions would be placed on the methods of evaluating testers' evidence.

HUD's original proposal would have established guidelines more in line with rules most private fair housing organizations use, such as accepting the testimony of a white tester and the home seeker who filed the discrimination complaint as evidence of discrimination.

Requiring two sets of testers could signal that an investigation is underway and is "tantamount to banks having to be robbed twice by the same robbers" before a case can be taken to court, said Lee Porter, director of the Bergen County, N.J., Fair Housing Council.

Courts accept the testimony of a home seeker and one tester as sufficient evidence in many cases, according to attorney Avery Friedman, who specializes in fair housing law. None of the estimated 275 housing discrimination cases tried in federal courts since the 1968 Civil Rights Act was passed could have been taken to court under the testing guidelines now proposed, he said. None of the judges "required this kind of evidence to establish that a violation" of law had occurred.

Although the restrictions agreed to by the administration and the NAR apply only to the testing funded under HUD's "private enforcement initiative," as the new program is to be known, civil rights groups fear they "may be misconstrued" as applying to evidence required in cases brought to trial under the 1968 civil rights law, Friedman said.

Limiting testing to cases of specific complaints "effectively guts fair housing testing," Martin said. Before the 1968 civil rights law was passed, discrimination was "open and advertised." Since then, "in Kentucky, we say we've gone to Northern-style discrimination . . . , polite, courteous and very devious." Testing of suspect landlords and sales people is essential if subtle bias is to be discovered, he said.

Knapp and North told the subcommittee they believe fears that courts will consider the guidelines as a standard for evidence in fair housing cases are unfounded.