Legislators representing 35 states, some stirred by black colleagues' appeals for support from "our white sisters and brothers," called today for "strict and immediate" economic sanctions against South Africa and the withholding of public funds from U.S. firms doing business there.

The 35-to-15 vote fell three short of the three-fourths majority needed for adoption as a policy recommendation of the National Conference of State Legislatures, the 11th annual meeting of which ends here Friday. But backers of the measure declined to concede defeat.

"We consider it a moral victory even though it was a technical loss," said Maryland state Sen. Clarence M. Mitchell III, president of the National Black Caucus of State Legislators. "The civil rights progress that we've made in this country has been made on moral victories."

The debate on the South Africa resolution dominated the business session, overshadowing the issue of federal tax revision that has become a growing concern of state and local officials throughout the country.

Before the South Africa debate, the organization easily approved a resolution that endorsed the principle of federal tax simplification. It also expressed "undiminished" concern, however, that a linchpin of President Reagan's tax-revision plan -- repeal of the deduction for state and local taxes -- would "adversely affect federalism and state and local revenue systems."

The support for tax revision was something of a victory for the Reagan administration. The organization's previous position, adopted by a committee in May, criticized some provisions of Reagan's plan and said nothing about tax revision in general.

The debate on South Africa took about two hours and when it was over, the 1,700 legislators had arrived at no official policy regarding that country. A move to merely condemn apartheid, postponing stronger recommendations pending a study mission, was rejected by critics, who termed it too weak and adopted a go-for-broke approach.

Apartheid critics, led by Alabama state Rep. Alvin Holmes, picked up support by deleting a section that called for severing diplomatic ties with South Africa and another that declared the system of racial separation "no less barbaric and demeaning to South African black inhabitants than were Hitler's atrocities."

Holmes evoked memories of black-white cooperation during the civil rights movement, and Pennsylvania state Rep. David P. Richardson Jr. noted that "everybody in this country is against apartheid. The only question that ever comes up is, 'But . . . ' " Richardson said, adding, "How much more can you hurt blacks when they're being killed every day?"