As the leader of Afrikanerdom made his way through the aisles of the huge tent holding 5,000 cheering National party supporters, there was a volley of thunder from the torrential electrical storm outside.

The perfect timing did not go unnoticed by the mostly middle class, middle-aged audience of Johannesburg residents attending the last mass campaign rally before South Africa's national elections Wednesday.

Dressed in a silver gray suit, Prime Minister John Vorster, followed by his wife, Tinie, in a blue chiffon dress, strode to the stage where they joined local National Party candidates, including the first Jewish paliamentary candidate running on the National ticket.

For 90 minutes he talked to the sea of all white faces, denouncing his enemies, especially the opposition Progressive Federal Party, which has branded Vorster's policies based on racial separation a failure. Gazing slowly around the arena, he deadianned: "If this is failure, I don't want success." They loved it.

Partisan buoyancy and bursts of applause, however, could not obscure South Africa's growing international isolation, the prospect of future economic difficulties and indications of increasing doubts about Vorster's apartheld course among the urban English speaking population.

Nevertheless, Wednesday's vote was expected not only to return Vorster's National Party to power, but to increase its 117 seats in the 165 member Parliament.

At the same time, however, polls indicated that the Progressive Party, which calls for dismantling of apartheld and the sharing of political rights with the blacks, may add several seats to the 13 it holds in the current Parliament.

The defiant leader launched into an attack on his opponents, starting with the English language newspapers that had endorsed the Progressive Federal Party.

Then he scoffed at the U.N. arms embargo, saying South Africa was self-sufficient to fight its enemies.

"We can deal with anything that comes out of Africa before breakfast," he said.

But Vorster saved his greatest vitriol for the progressive Federal Party whose position is that majority rule must eventually come to South Africa. Vorster's eyes narrowed in scorn at this apostasy in Afrikanerdom - the abadoning of apartheld.

Majority rule "will affect your standard of living, your lives," Vorster told his backers. It would mean integrated neighborhoods and integrated schools, he reminded the all white audience.

"No. No way. Never! they shouted back. Choose the party that "will guarantee your security," Vorster told them.

The big question mark in Wednesday's vote is which way the English-speaking bloc will go now that their traditional United Party has dissolved. The English-bloc comprises roughly 40 per cent of white South Africans, as against the majority Afrikaners, who are solidly behind Vorster.

Various polls indicated that the support for the National Party among urban voters has decreased sharply during the past two weeks while the Progressives had picked up strength.

The outcome of the vote in some key urban areas, where opposition candidates normally did well against the National Party, may be affected by the New Prepublic Party, which was formed after the collapse of the United Partly.

Also here tonight, about 3,000 younger, affluent and mostly English-speaking urbanites crammed into City Hall for the final Progressive rally. The keynote there, too, was security - but "security through negotiations."

Helen Suzman, for more than a decade the lone parliamentary member of the Progressive Party, declared that "it's time every voter let his conscience speak."

"Let it speak against detention without trial against bannings and against interference with our free press . . . against the bulldozing of shacks and . . . against inferior education for blacks . . . against a government that has failed on all counts."

Suzman called on her Progressive supporters to reject the National Party's campaign slogan - "go forward together" - because "it will be forward together over the cliff and into the sea, like a lot of demmings."

Political observers and polls indicated that the Progressive Party is likely to emerge as the official parliamentary opposition after Wednesday's vote. Thus, political attitudes toward Vorster's apartheid policies among the 2.1 million eligible voters are expected to lead to a polarization - however modest - in the parliament reflecting the white population's divisions on this crucial issue.