It was a low point for black American leaders.

Ronald Reagan had been reelected in a landslide despite near-total black opposition. The Democratic Party was focusing on recapturing white male support. Jesse L. Jackson's emotional campaign for president was over, with no tangible gains. Budget cuts were approaching in an already hostile atmosphere for poverty programs.

"It was like blacks had leprosy," said Rep. Ronald V. Dellums (D-Calif.) "Republicans and Democrats were moving away from blacks."

Into that dreary climate has come a sudden flash -- the "Free South Africa Movement." In sharp contrast to years of hazy, inconclusive discussions over causes identified with blacks -- busing, quotas and poverty programs -- the demonstrations at the South African Embassy here and at consulates in cities around the country have attracted substantial white support, including labor unions and even conservatives.

"We were down so far we had to do something -- you can't fall off the ground," said Randall Robinson, executive director of the foreign policy lobby TransAfrica and the leader of the demonstrations against apartheid.

After years of looking for lightning to strike -- the right issue at the right time to revive the moribund civil rights movement -- black American leaders have found the issue in apartheid. And it has come under Robinson's leadership, with the help of D.C. Del. Walter Fauntroy. Both men were arrested on Nov. 21 in the first of the series of protests at the South African Embassy.

Robinson has tackled the issue with a sense of timing and a sense of the right opponent.

While Richard Hatcher, the mayor of Gary, Ind., and the chairman of TransAfrica's board, had considered a sit-in in the White House driveway to protest worsening apartheid in South Africa, Robinson said he never considered that option. Instead of focusing on the popular Reagan, he took aim at the South African embassy.

"There was a lull after the election and before Congress convenes, it's the Christmas season, and the press really picked this up," said Rep. Julian Dixon (D-Calif.).

The presence of Bishop Desmond Tutu in the United States, on his way to accept the Nobel Peace Prize, spurred the demonstrations, as did South Africa's jailing of black labor leaders. And the tactic of street demonstrations, reminiscent of the old civil rights days, startled politicians and press alike into paying attention, several black leaders said.

Another factor in the movement's success is Robinson's low-profile presence, which has neither antagonized white political leaders nor prompted jealousies within the black leadership, according to several black leaders.

There has been some friction surrounding Jackson's role and his reputation for scene stealing. But by and large, black congressmen, religious leaders and civil rights leaders, split during the presidential campaign and in a sense rivals for prominence since the civil rights movement, have commonly targeted South Africa as a major issue and followed TransAfrica's lead on how to address the problem.

"The time was right, the people were willing and now it is a national movement," said Mary Frances Berry, a member of the U.S. Commission on Civil Rights.

Critics, including some within the Reagan administration, have charged that black leaders are ignoring domestic issues to concern themselves with events in South Africa.

"What about the budget? What about taxes?" asked Steven J. Rhodes, assistant to the vice president for domestic policy. "If you want to be in the mainstream of power, that is where you've got to be. Regardless of what we do in the United States, the South African government will determine what happens over there. Black leaders could have more impact in directing federal spending to people who depend on those programs."

Clarence Pendleton, chairman of the Civil Rights Commission, has asked if American blacks are able to weigh U.S. interests in a stable South African government against their concern over apartheid. Does "this pigment attachment come first?" he asked.

But so far, the criticism has not slowed the movement.

South Africa represents a point of common concern for groups working on issues besides racism -- from nuclear freeze advocates concerned that South Africa has or soon will get the bomb, to human rights groups.

"Many of the people I'm walking with on the picket line are people who want to say something to the Reagan administration," said Roger Wilkins, a writer and former assistant attorney general. "They don't like the constructive engagement policy of the administration, but they also don't like the general thrust of the administration and this is their chance to speak to Reagan . . . . Even Reaganites are out there saying, 'I'm for the president but I can't stomach South Africa.' "

The movement also has given conservative congressmen, including 35 House members who sent a letter to South African Ambassador Bernardus G. Fourie threatening economic or other sanctions, a chance to join with blacks in a cause and undercut charges that their opposition to blacks on other issues is racially motivated.

Jackson, by his attention to foreign policy in general and South Africa specifically during his presidential campaign, had set the stage for an outburst of black concern about a foreign policy issue.

But several black leaders, speaking privately, suggested ironically that the reason the movement caught fire dramatically was that at the time of the first arrests, Jackson was in Chicago, bedridden with bronchitis.

"You've got to remember ten days earlier Jesse led a demonstration at the embassy and the papers paid no attention," said a black leader who asked to speak without being identified. "Without Jesse, this had the look of an old-time civil rights uprising and there were no personalities involved. There is no question which side is the good side."

Jackson points to Reagan's reelection as the catalyst that caused the movement to crystallize, forcing people to work outside the political system for change.

"Interestingly enough," he said, "I think if Mondale had won, many of the black [congressmen] who are now picketing would have been working on their committee positions and been willing to use the process. They've gone outside, though . . . . It is difficult to tell what's going to be a hit. You don't do your best every time you cut a record, you know."

The "hit" has reshuffled the relationships among black leaders in the aftermath of Jackson's campaign. Suddenly Fauntroy and Robinson have been elevated into the national spotlight and Jackson cast as a supporting actor only months after he'd held the leading role.

Fauntroy reportedly angered Jackson several months ago with his attempt to win the top post for blacks in Walter F. Mondale's presidential campaign, and Jackson reportedly blocked Fauntroy's bid. Now, sources describe the relationship between the two men as a wary one, with Fauntroy trying not to let the personal problems divert attention from the South Africa issue.

According to several sources, Robinson and the black congressmen who have led the movement are wary of Jackson's tendency to take over any stage, although they are striving publicly to play down any tension.

They are particularly concerned that Jackson may travel to South Africa, and possibly gain the release of some jailed black South Africans. That dramatic gesture might unintentionally bring their budding movement to an abrupt end before concrete changes are made in the Reagan administration's South Africa policy, they say.

In a meeting with members of the Congressional Black Caucus last Wednesday, Jackson was told he has to recognize that South Africa granted him a visa because of the pressure from the demonstrations, sources said. And he was told he needs to coordinate the timing of his trip to South Africa with other members of the movement to serve the goal of building momentum for the cause in the United States.

At the risk of being isolated as what one black congressman called a "loose cannon," Jackson agreed to join the steering committee of the "Free South Africa Movement" as one of about 10 members operating under Fauntroy's and Robinson's guidance. Jackson also received a pledge from the Black Caucus to help retire his campaign debt.

"I understand all the concern over Jesse's role," said Hatcher, who was chairman of Jackson's campaign. "But in football terminology it would be a serious mistake to have your best player sitting on the bench . . . . He may not have started the game, but you've got to get the star in the game."