For a rare moment yesterday, the cavernous floor of the House of Representatives looked like a genuine racial melting pot, the people's body it is supposed to be. The number of black, yellow and brown faces almost equaled the number of white ones.

It so surprised Rep. Robert Garcia (D-N.Y.), who represents the ghettos of the south Bronx, that he stopped in the middle of a heated debate and assured visitors in the galleries that the Congress of the United States isn't "overwhelmingly minority."

"We're all here today, and we're here because we're fighting for a fundamental right that affects every one of our communities," he said in a deep, resounding voice.

The debate was over extension of the Voting Rights Act of 1965. It was a highly symbolic one for Garcia and many others. "The fact is I am a product of the act," he said.

The act is considered, by friends and foes alike, the most successful piece of civil rights legislation in U.S. history. With budget cuts hitting minority groups particularly hard, the matter of its extension has taken on an added sense of urgency this year.

The sounds from the House floor reflected that. Rep. William H. Gray III (D-Pa.) said there is a "new politics of the 1980s" that has "a blatant disregard for the disadvantaged."

"It is an economic disaster for poor and minority families," Gray said.

Rep. Parren J. Mitchell (D-Md.) said "the climate we find ourselves in" gives the bill "enormous significance." Budget cuts and "attacks on affirmative action," he said, have created "fear in the black community."

All this was somewhat off the direct point of argument as the major civil rights debate of the 97th Congress opened, but the politics of 1981 are indisputably different from the politics of 1965, when there were only five blacks in Congress. White southern Republicans now join black Democrats in praising the act.

The central question for the House, which will vote on the matter Monday, is not if there will be an extension, but what kind of an extension. Republicans warned that they will seek major changes in a version of the act approved by the House Judiciary Committee; Democrats argued that any change would cripple it.

The strongest and most pointed challenges, Democrats said, would come in two areas: bilingual requirements and "bailout" procedures for local and state governments in 24 states, most of them in the South, that are covered by the act.

Under the bailout provision, jurisdictions that have been in compliance with the act for 10 years would be exempted from the need to get Justice Department approval for any change in their election proceedings.

The Reagan administration has yet to take a position on these provisions, although it had been scheduled to receive a report on them Thursday.

The opposition yesterday was led by Rep. M. Caldwell Butler (R-Va.) and Rep. Henry J. Hyde (R-Ill.), who said the bailout provision "is the device by which the South will be permitted to rejoin the Union."

Butler, who claimed Virginia would not be able to qualify for such a bailout for 10 years, came under bitter attack from several Democrats, who accused him of being an obstructionist.

"Mr. Butler is one whose tongue is smoother than butter on civil rights, but there is war in his heart," said D.C. Del. Walter E. Fauntroy, a Democrat.

Democrats claim that one-fourth of the almost 8,000 jurisdictions covered by the act would be able to bail out of its provisions in 1984. But they argued that hearings on the measure found continued widespread abuses, designed to continue discrimination against minorities.

Rep. Harold E. Ford (D-Tenn.) said blacks constitute 25 percent of the population in the Deep South, but hold only 5.6 percent of the political offices. Only 3 percent of the state senators, 8 percent of the state representatives and 3 percent of the mayors in the region are black, he said.