Johannesburg — The far right has won its first political beachhead against the Nationalist government of South Africa since the Nationalists came to power in 1948. And if the past is any indicator, this is likely to have a retarding influence on the ''reform'' program of Prime Minister Pieter W. Botha, seasoned political analysts here say.
In three parliamentary by-elections May 11, the ruling National Party took a thumping in one, scraped home in another, and won a comfortable victory in the third.
The main issue in the by-elections was Botha's so-called ''reform'' plan, which calls for the admission of Coloreds (persons of mixed-race descent) and Indians into the now all-white Parliament. Blacks would continue to be excluded.
Most significant was the evident weakening of support for the National Party in two rural constituencies in the northern Transvaal Province - considered the ideological heartland of the white Afrikaner population. The two constituencies were Waterberg and Soutpansberg.
In Waterberg, the leader of the Conservative Party, Dr. Andries Treurnicht, grabbed what had traditionally been a safe National Party seat. Dr. Treurnicht formed the Conservative Party last year out of dissatisfaction over Botha's reform plan.
And although Dr. Treurnicht and 17 other National Party breakaways had remained in Parliament under the Conservative Party banner, this marks the first electoral victory of a right-wing opponent of the Nationalist government.
Dr. Treurnicht's win by a large margin enhances his stature and gives the right wing a ''definite political power base'' from which to operate, says Andre du Toit, a professor of political philosophy at the University of Stellenbosch.
In Soutpansberg the government minister of manpower, Stephanus Botha, retained his seat in Parliament, albeit by a much smaller margin than he obtained in the 1981 general election. He beat his Conservative Party opponent, Thomas Langley, by 621 votes.
In the Waterkloof by-election - an urban constituency in Pretoria - the National Party coasted home to an easy victory. Its main rival was the left-of- government opposition Progressive Federal Party.
The strengthening of the right wing underscores the polarized nature of politics in South Africa. Many critics to the left of the government - which when nonwhites are included far outnumber those to the right - already consider Botha's ''reform'' plans ''too little, too late.''
Many of those to the left of the government who support the ''reforms'' do so because they see it as less bad than doing nothing.
The prime minister said in a statement: ''We realize the voting public is deeply divided, as has become apparent in these by-elections.''
The government has introduced legislation to implement its ''reforms,'' but its fate is uncertain. The government earlier promised at some stage to test white, Colored, and Indian opinion on the reform plan. Botha's statement also said ''the government will have to make an in-depth study of this (the by-election results) and will have to weigh up properly the implications.''
In many respects, the by-elections confirmed a strengthening in right-wing sentiment that was evident in the 1981 general election.
Professor du Toit says the by-election offers the Nationalist government two options that have been pressing in for some time: Forget about winning back the right wing and press ahead with its version of reform, or go all out to reunify the Afrikaners behind the National Party. He says the first option appeals to the head, and the second to the heart.
''If past experience is any indicator, the heart will win over the head,'' he reckons.