For Sen. Richard G. Lugar (R-Ind.), April 4, 1968, was a day that sticks in memory.

It was his 36th birthday. It was the day that Robert F. Kennedy campaigned for the Democratic presidential nomination in Indianapolis, where Lugar was the newly elected mayor. And it was the day that the Rev. Martin Luther King Jr. was assassinated.

King's murder set off waves of violence that swept through many cities, including the nation's capital. But Indianapolis was spared, in part, Lugar said, because he and his administration moved swiftly to reach out to the city's black community.

"It was a time of enormous division," he said. "I spent days and weeks on street corners and in church basements, talking about justice and coming together. It was really a laying on of hands. We weren't going to have much of a city if we weren't successful."

On Tuesday, Lugar, now the chairman of the Senate Foreign Relations Committee, will attempt to reach out symbolically to another black community. Defying President Reagan, the Republican-controlled committee will begin drafting legislation to impose new economic sanctions on the white minority government of South Africa.

Lugar calls his term as mayor -- and earlier service on the Indianapolis School Board during the height of the school desegregation controversy -- "critically important in my own education."

These experiences have helped shape his outlook on South Africa. And in a larger sense, the whole American experience with the politics of race, from the sit-ins of the 1950s to the urban violence of the 1960s, is helping to shape and drive the congressional momentum toward sanctions legislation.

This became clear last week when Secretary of State George P. Shultz appeared before the Foreign Relations Committee to defend the president's policy. An economist by training and believer in the invisible workings of the market place by inclination, Shultz argued for a cautious approach.

The market, Shultz said, is imposing sanctions on South Africa, where the economy is in a steady slide brought on by the violence and fear surrounding the struggle to end the apartheid system of strict racial segregation. U.S. firms, western capital and skilled white South Africans are fleeing the country, he said. The imposition of sanctions from the outside, Shultz argued, might serve only to "dilute" the impact of these inexorable economic forces.

Whatever the economic merits to these arguments, they ran head-on into a political imperative that is strongly at work on Capitol Hill and which is rooted in part in the U.S. civil rights experience: That at a time of crisis and increasing televised violence, to do nothing -- waiting for the "market" or some other unseen force to work -- is to invite political disaster.

It was no contest. Before Shultz began his testimony, Sen. Christopher J. Dodd (D-Conn.) quoted to him from King's letter from a Birmingham jail. Sen. Joseph R. Biden Jr. (D-Conn.) derided the president's speech of the day before for its "echoes of the past," when numerous arguments were advanced for a go-slow approach to desegregation.

Under questioning, even Shultz fell back on the values and imagery of the civil rights movement. Asked by Sen. Paul S. Sarbanes (D-Md.) if he would renounce violence if he were a young, black South African, Shultz replied:

"I admire very much the leadership Martin Luther King gave to our own civil rights movement. I recognize our countries are different, but I like to think the nonviolent mode is the way to go."

"It is an impossible comparison to avoid," Sen. Charles McC. Mathias Jr. (R-Md.) said, after the hearing, of the parallels between the U.S. civil rights movement and the struggle in South Africa. It can also be a "dangerous" comparison because of the vast differences in the two situations, Mathias added, but that does not diminish the political imperative facing Congress.

"The situation is deteriorating very fast," he said. "We really have to do something."

Adding to the pressure is concern among Republicans that the increasingly unpopular Reagan policy will damage their party, widening the gulf that separates the GOP and the vast majority of black Americans.

"I think it will rub off badly if we don't take some action," Mathias said.

Some Republicans tried to derail the drive for sanctions. In the House, a handful of conservatives, apparently acting on their own, did not contest the passage of far-reaching legislation sponsored by Rep. Ronald V. Dellums (D-Calif.) that would impose an almost total U.S. trade embargo on South Africa and require U.S. companies to withdraw investments there.

The conservatives' theory, voiced at the time by Rep. Mark D. Siljander (R-Mich.), was that the Dellums legislation was so "extreme" that it would be buried in the Republican-controlled Senate, ending the sanctions issue for this year.

Siljander and the others badly miscalculated. In the Senate, the Dellums measure is being sponsored by Sens. Alan Cranston (D-Calif), Edward M. Kennedy (D-Mass.) and Lowell P. Weicker Jr. (R-Conn.). Its strong Democratic backing only adds to the pressure on Lugar, Senate Majority Leader Robert J. Dole (R-Kan.) and other GOP lawmakers to produce legislation that goes well beyond Reagan's rhetorical denunciations of apartheid.

The White House, too, is showing signs of buckling under the pressure, suggesting a willingness to consider some sanctions against the Pretoria regime. This is what happened last year, when Reagan preempted Congress by signing an executive order that incorporated many of the provisions of a sanctions bill that was on the verge of passage.

In the more charged atmosphere of this summer, such a tactic may come too late if it is tried at all. "It would have to be a very far-reaching executive order" to prevent Congress from passing a sanctions measure, said Sen. John Heinz (R-Pa.).

At last week's Foreign Relations Committee hearing, it was Biden's emotional, table-pounding clash with Shultz that attracted the most attention. But the most significant utterance of the day may have come from Lugar, who quietly told his colleagues, "We will have a markup" to draft legislation.

The former mayor of Indianapolis had made up his mind. Later in the week he was confronted by a questioner who suggested this was all an example of "feel good" diplomacy, designed for domestic consumption but with little or no chance of achieving its objective.

"People who make those statements really don't appreciate how deeply the plight of South Africa is felt in the black community," Lugar said. "And I'm not saying it's only felt in the black community. Young people on the campuses, people with deep religious feelings, others are in anguish when they watch television and see the plight of South Africa.

"Our foreign policy should address our own anguish and our own history."