If there were such a thing as a Missing Political Persons Bureau, one of its files might read: Afrikaner, 65. Best known for advocating change to avoid revolution in South Africa. Has fuzzy vision of future, but heart believed by many to have been in right place. Thought to be weakening in commitment to change. Name: Pieter Willem Botha.

When Parliament convened here early this month, for the first time since the all-white election in April, this man was nowhere to be seen or heard.

In his seat at the speaker's right hand, the place traditionally reserved for the leader of the ruling National Party, was another Pieter Willem Botha.

This was not an advocate of change in South Africa's apartheid policies, but a man who had returned to the shopworn white supremacist ideology of the National Party. It was a man desperately trying to win back the 200,000 right-wing voters who rejected, by an unexpectedly large margin, proposed changes in South Africa's system of white-minority rule.

Under attack in the parliamentary debate from opposition leader Frederik Van Zyl Slabbert, Prime Minister Botha reacted to the lesson of the April election by saying urban blacks must exercise their political rights only through the tribal homelands. He ruled out a common voters' roll of whites, Indians and Coloreds (the official South African designation for persons of mixed race) and a common parliament.

When Slabbert said that the National Party's "concept of white self-determination is none other than white domination," Botha snapped back, "In this state, yes."

The small but hopeful phalanx of moderate Afrikaners who were geared up to be Botha's storm troopers of change must have blanched as they read the newspaper accounts of the debate. Many must have felt like the forward scout who receives a message from headquarters: "Mission aborted, general resigned."

Commiserating with one of the more moderate, change-oriented National Party members of Parliament last week, a Western diplomat asked him when he was going to join the opposition party. "Well, tonight I'm still in the National Party, but I don't know about a year from now," he replied.

It would be wrong, however, to assume that the National Party is about to break up or that a realignment of white politics is imminent. Many more economic and political pressures are needed for that. Such a development will occur, Slabbert observed, only "when it becomes apparent that to maintain domination, they need to sacrifice their life styles, to pay more to keep it and to fight for it."

What does seem apparent is that the right wing of the National Party, after a brief period of disarray under Botha's leadership, has been emboldened by the April election and has the upper hand again.

Even before Parliament opened, it became apparent that a priority of the Cabinet is to win back the disaffected electorate. Black Affairs Minister Pieter Koornhof, who in 1979 told a Washington audience that "apartheid is dead," disclosed recently that a special committee has been set up to investigate the "crowding out" of whites by blacks in urban areas.

In a newspaper interview he said, "When 90,000 blacks have to cross the same bridge with 1,000 whites every day, unpleasant incidents can occur. It can be that it may be a solution for each to have his own bridge . . . in the interests of good order."

That prompted opposition member Horace Van Rensburg to describe the National Party's policy as "one man, one bridge."

Some observers believe Botha's attempts to recoup his losses are going to prove futile.

"History shows," writes Afrikaner historian Hermann Giliomee, "that once Afrikaners have turned against a leader their alienation is irreversible."

Some press commentators have even gone so far as to say Botha's implicit admission of weakness in relation to the right-wing faction of his party leaves him a lame-duck leader in his party caucus.

There are those within the National Party who argue that Botha has simply adopted new tactics, that instead of advertising his plans and giving his right wing time to marshal its forces, he is keeping his plans to himself.

"I have my own way of dealing with things," he said.

If that is so, he is just delaying the inevitable showdown, because all constitutional changes recommended by the advisory body called the President's Council, which Botha set up, must be accepted by the National Party's rank-and-file membership.

Compounding Botha's problems is a worsening economic situation. After a golden year in 1980, when its economy grew 8 percent, South Africa is headed for tougher times with a falling gold price, 15 percent inflation and a downturn in growth to 41/2 percent, with an expected rate of only 2 percent next year.

Botha's delicate position does not augur well for domestic change or for a settlement of the dispute over rule in the territory of Namibia (Southwest Africa).

Instead there is this comment of Slabbert to ponder, "We should be involving other racial groups in a debate on how we can live together. But if you believe as the prime minister does that white domination is nonnegotiable, then we have lost the battle before it is begun. I fear a state of almost semipermanent siege in this country, similar to the situations in Lebanon and Northern Ireland."