In an unprecedented political move, South Africa's oldest and largest official opposition movement, the United Party, voted to dissolve today in a step that opens the way for a major realighment of white opposition forces.

At a special congress here, United Party leader Sir de Villiers Graaff called on the moderate political body to end its 29-year struggle to recapture the leadership the party held between 1934 and 1948 and make way for a more vigorous movement to counter the dominant National Party of Prime Minister John Vorster.

Since 1948, when it was unexpectedly defeated by the nationalists, the United Party has been increasingly troubled, mainly because its fragmented power base made clear-cut policy and hard-line opposition next to impossible. Nicknamed the "Party of National Reconciliation," the United Party has had trouble reconciling its own members, repeately suffering leadership splits and serious voter losses.

The congress also voted to merge with the fledgling Democratic Party at a convention here Wednesday to form what will probably be called the United Confederal Party.

Sir de Villiers outlined the new party's basic principles as:

A thrust away from discrimination.

Consultation and negotiation between racial groups.

Establishment of a multiracial central structure representing all groups.

Maximum practicable self-rule for all.

He said the principal objective would be to improve the general climate of race relations immediately by removing "justified grievances." He added, however:

"We must not move too far left, nor too far right. We must be a centrist party, which will avoid polarization . . . We are not prepared to accept the policy of one-man, one-vote, which will set so low a ceiling that we can never become an alternative government."

Political analysts from all viewpoints are already predicting, however, that the new party is doomed. Minister of Interior and Information Connie Mulder called its platform "vague" and "just old UP policy dressed up a bit with a few adaptations to meet new circumstances, such as the political position of the urban blacks.

"I do not fear it and I cannot see any nationalists being interested in such a vague, unpredictable future," he added.

The leader of the progressive Reform Party, Colin Eglin, called the effort "disappointing" since it failed to provide a political home for many thousands of voters who favor a clear-out commitment to move away from race discrimination.

Ironically, analysts are also predicting that the new party may accomplish its goal of a move vigorous opposition by self-defeat, since there already appears to be resistance among United Party members to join it.

Members might realign with the Progressive Reform Party and BOOST the efforts of South Africa's most moderate white group. At least six United Party members of Parliament are scheduled to talk tonight with reform leaders about a merger. As Johannesburg's Rand Daily Mail newspaper pointed out today:

"As far as their own interests are concerned, the UP leaders may be making a monumental blunders, but in doing so they are performing a national service because they are breaking up the logjam which has blocked any forward movement in our national affairs for more than a generation."

The UP has 36 seats in the 171-member Parliament, while the Reform Party has 12.

The United Party, founded in 1934 by former Prime Minister Jan Smuts, has traditionally been stubborn about moving either to the right or left. Originally it was to bring together the Boers (early Dutch settlers) and rival British settlers, farmers and city dwellers, industrialists and workers - such a wide constituency that the party was forced to adopt a nebulous platform.

The strong stand of the Afrikaner dominated National Party on race issues led to the United Party's downfall in 1948. During every subsequent political test, when the United Party had to take a stand, one of its wings broke off - including members who later formed the Progressive Reform Party.

Since Sir de Villiers took over its leadership more than 20 years ago, the United Party has been criticized for its refusal to offer any strong opposition or imaginative policy alternative to the Nationalists' plans for apartheid - separate development for separate races.

The United Party has been so divided that there was even a heated debate today over whether the party should dissolve, with nine members of Parliament voting against the move.

Sir de Villiers, 63, announced that he would step down from the leadership to give the new party a fresh image. Even this is not expected to help keep efforts alive for very long.