JOHANNESBURG -- A gradual narrowing of the wage gap between South Africa's working-class whites and unionized blacks is likely to have broad implications for the future of the ruling National Party, according to governmental and private economists.

The beneficiary of the wage trend is likely to be the far-right Conservative Party, which has been increasing its constituency. The party has been reaching out from the rural areas to the blue-collar suburbs in the industrial belt that stretches from Pretoria to the Vaal River south of Johannesburg, generally following the line of coal and gold seams, the economists say.

Moreover, wage restraints and hiring cutbacks implemented as part of President Pieter W. Botha's new policy of fiscal discipline could exacerbate the lower-income whites' plight and lead to an electoral backlash against the Nationalists, according to economic analysts in the party.

"It's a genuine problem. When you spend heavily to improve the quality of life for one group, it makes it tighter for others," said Frederick du Plessis, one of Botha's key economic advisers and chairman of the Sanlam insurance and property conglomorate.

Du Plessis was referring to plans to spend billions of dollars in a "new economic dispensation" for blacks that involves a massive housing program, improved education and the creation of job opportunities. He said restiveness among working-class whites could cost the National Party votes in next year's scheduled whites-only election for Parliament.

Earlier tests of the backlash factor will come next month in three important off-year elections for seats in Parliament and in nationwide municipal elections scheduled for October.

In last year's whites-only election, the Conservative Party won 23 seats, gaining new status as the official opposition party in Parliament.

While the country's 85 percent black majority still trails far behind whites in income -- with a $175 monthly average compared to about $1,000 for whites -- the gap between working-class whites and mostly unionized black industrial workers is slowly closing.

In industry, black wages have, for the most part, kept pace with inflation, while white wages have fallen short by about 4 percent yearly. The current annual rate of inflation is 14.7 percent, down from more than 20 percent a year ago.

In the textile industry, black wages increased nearly 18 percent between 1983 and 1984, while whites' wages increased only 11 percent, according to the government's Central Statistics Services.

In the construction industry, blacks' wages grew 12.7 percent between June 1984 and June 1985, while wages for whites increased only 5.3 percent.

In the civil service, the 141,000 black workers in the central government saw their wages rise 24 percent between 1984 and 1985, while the 150,000 white workers' increased only 6 percent.

A major factor in the narrowing wage gap, economic analysts say, has been the expanding black union movement, while the number of exclusively white unions fell from 80 to 59 between 1980 and 1984, according to the National Manpower Commission.

The black unions' militancy has resulted in the repeal of most job reservation laws, which protected white workers from black infringement in selected job categories, thereby forcing employers to raise the wages of blacks.

For example, scrapping job protections in the mining industry meant that black mine workers, who had been limited to drilling holes for dynamite charges and hauling ore, could get higher-paying jobs as blasters.

In the civil service, an eighth-grade high school certificate traditionally could get a white a job in the post office and assure him of patronage protection. By 1985, however, there were 41,000 blacks in the Posts and Telecommunications Department, compared to 53,000 whites, and their wages had increased by 40 percent from the previous year, compared to an 11 percent increase for whites.

Postal officials say they are still seeking to attain parity between white and black wages. Whites are accepting more jobs normally done by blacks, such as service station attendants. A consequence of the closing of wage gaps and other economic changes designed to improve the economic status of blacks is that many working-class whites feel they are being sacrificed on the altar of upper middle-class morality.

Thomas Langley, a member of Parliament and spokesman for the Conservative Party, said that historically there was no class system within white Afrikanerdom. Under the National Party's economic policies, however, he said there is a danger of widening the gap between a layer of affluent whites and a growing working class that finds it difficult to make ends meet.

"I think there is a danger of a class system growing more defined among whites," Langley said.

The resentment of working-class whites, economic analysts say, stems not so much from unemployment -- which is still less than 2 percent for the white labor force, compared to an estimated 40 percent for blacks -- but from a perception that the government and white big business have formed a cabal to placate black unrest with expensive programs that will increase the economic squeeze on lower-income whites.

Compounding their fears is a belief that if proposed relaxations in housing segregation laws are adopted, their blue-collar neighborhoods would be the first to attract blacks, despite Pretoria's assurances that local communities will be able to veto open housing.

The government's recent announcement that it plans to privatize state-owned businesses, including transportation, postal and telecommunications services and the steel industry, also is likely to give rise to fears by workers that the new managements will try to cut costs and increase profits by laying off employees. Officials insisted they will protect the jobs of these workers.