Both the white-minority government here and its most ardent opponents agree that the South Africa sanctions bill that the U.S. Senate will take up when it returns Monday will have little if any direct economic effect.

But the psychological impact of the bill -- the dread it has created among government officials and supporters and the expectations it has kindled among many of South Africa's black majority -- has helped transform a modest piece of legislation into a symbolic turning point in U.S. relations with this troubled country.

Both sides here see the bill as a major step down a long trail of increasing economic pressures on the Pretoria government that already have included investment bans by eight states and a number of American cities and a refusal by banks to make new loans. It is a trail that many believe will lead ultimately to a total embargo on trade and investment.

"The American decision on this issue may just start a train of events which America will find impossible to control," Deputy Foreign Minister Louis Nel warned in a press conference Thursday.

As a result, the bill has raised political and moral questions about the economic damage that more stringent measures could do to blacks both inside South Africa and in neighboring black-ruled states that rely on this country for a significant share of their commerce.

President Reagan repeatedly has cited this issue in expressing his opposition to any sanctions, contending that such measures "would hurt the very people we are trying to help." He has argued along with many businessmen that continued U.S. investment creates new job opportunities for blacks and ultimately helps enhance their economic clout.

But much different views can be heard from South Africa's disenfranchised black majority. While few black leaders deny that sanctions would hurt blacks, there are deep divisions over whether the economic pain is necessary to force a recalcitrant government into significant reforms -- and growing indications that, as political lines harden further between black and white, more blacks are willing to make the sacrifice.

Zulu Chief Gatsha Buthelezi, a political moderate who leads the million-member Inkatha political and social movement, has argued strongly against sanctions, contending that they harm blacks more than whites and could help wreck an economy blacks will inherit someday when white-minority rule ends. "We need the jobs," he says.

Just a year ago that view seemed to have strong support among blacks, according to a survey conducted for the State Department by a University of Natal social scientist, Lawrence Schlemmer, the country's leading academic pollster.

Polling 550 black factory workers in major industrial centers where multinational investment is concentrated, Schlemmer found that although 66 percent of his respondents characterized themselves as "unhappy and angry" with life in South Africa, only 25 percent supported sanctions such as a trade boycott or withdrawal of investment. The majority of those opposed cited the loss of jobs and harm to blacks as their main reasons.

But a Gallup Poll of 400 blacks conducted last month gave a far different picture: it showed 77 percent favored economic sanctions "unless South Africa agrees to get rid of the apartheid system."

Some of the discrepancy, Schlemmer said in an interview, may be due to different wording in the polls. Nonetheless, he said, "It's certainly true there has been a shift among blacks in favor of sanctions, although whether or not a majority would endorse specific steps like a withdrawal of capital or a trade embargo is an unknown factor at this point."

Black leaders to the left of Buthelezi generally reflect the view expressed in the Gallup Poll. Desmond Tutu, the Episcopal bishop of Johannesburg and a Nobel Peace laureate, has called repeatedly for increased economic pressure on Pretoria, although he has avoided a call for specific measures, in part because such a statement would violate South African law.

Similarly, the United Democratic Front, a coalition of 700 antiapartheid commumity organizations, said in a memorandum delivered to western embassies last month: "Your continued participation in the South African economy sustains apartheid and encourages the belligerent and violent policies of the government."

Buthelezi scoffs at such talk, contending that "90 percent" of those black leaders who advocate sanctions would themselves not be affected personally.

A South African diplomat said blacks will bear an inordinate share of the burden of any sanctions because they are usually the last hired and first fired. He bluntly stated his view of the possible consequences: "You may have noticed that this isn't Sweden. Each black worker supports at least six other people with his income. When you lose your job, people go hungry."

The diplomat is one of many analysts here who insist that sanctions will only make the government more defiant. They point to the example of the international arms embargo that has helped stimulate Pretoria into developing its own weapons industry. But others contend that outside economic pressure is one of many factors pushing the government toward change.

The U.S. Anti-Apartheid Act of 1985 actually would do little to end U.S. participation in South Africa's economy and would cost few if any black jobs in its present form.

The only new sanctions in the bill are bans on U.S. bank loans to the government except for nondiscriminatory housing, health and education projects, and on U.S. sales of South Africa's gold Krugerrand.

American banks had nearly $4.6 billion in loans to South Africa outstanding at the end of 1984, according to Federal Reserve Board figures, but only $374 million of that was to the public sector, less than the 1978 level. Most major U.S. banks have announced bans on loans to the Pretoria government.

The bill would tighten existing restrictions against the export of nuclear technology to South Africa and the sale of computers to the military, police and government agencies involved in the enforcement of apartheid.

It also would provide millions of dollars annually for black scholarships and a human rights fund and make mandatory an equal-employment code, known as the Sullivan Principles, for U.S. businesses operating in South Africa.

It would pose the possibility of sanctions against new investment, a denial of most-favored-nation trading status, and a ban on imports of South African coal and uranium if Pretoria fails to improve.

Some opponents of sanctions say the bill in its present form would be so mild that they are hoping for its speedy enactment.

"It would advisable for Reagan to sign this, get it done and get the heat off," said Lionel Gruen, assistant vice president at Citibank here and national coordinator of the Sullivan Principles. "Imagine if you throw this bill out and the South African government in its customary fashion does something stupid. Then the pressures for something really drastic would be enormous."

But Gruen's view is a minority one. Most businessmen working for American companies here agree with the South African government in seeing the bill as a long-term threat that could inspire disinvestment campaigners in Europe and lead to U.N. sanctions.

At his work bench at Rank Xerox's Johannesburg headquarters, David Matlala, a young black technician, offered a different opinion. Matlala has a wife and son, bills to pay and a job with one of the area's better-paying firms, but he said: "I am prepared to join the ranks of the unemployed if that is what is necessary."

Besides, he said, "whites are the ones with all the privileges, and they are the ones who will suffer the most. Blacks are used to hardships, so if sanctions will speed up things, it's worth it."

Matlala's view is similar to that of blacks in much of the plant, owned by a British firm that in turn is 51 percent owned by the Xerox Corp. in the United States. It is also the view of the predominantly black Federation of South African Trade Unions.

Alec Owen, education secretary for the 150,000 member federation, said, "The main thing hurting black workers in this country is not disinvestment but the internal economic and political policies of the government. We believe in maximum outside pressure because we don't think the government can ignore it."

Nonetheless, the federation draws the line at foreign firms actually pulling plants out of South Africa. "Those factories were built up by South African workers through accumulated profits, and we think they belong here," Owen said.

Wellington Mnikati, employe relations manager at Rank Xerox, said: "I support putting on pressure, but the question is how much and where do you stop, because if you go too far and companies start to pull out, the whole thing can break."