JOHANNESBURG, MARCH 3 -- Commanding victories by the extreme right-wing Conservative Party in parliamentary by-elections in two Transvaal Province districts yesterday are likely to set the tone for rural politics in South Africa for years to come and could affect the pace of the government's agenda for political and economic reform.

By itself, the voting in Standerton and Schweizer-Reneke, deep in the conservative heartland south of Johannesburg, had no effect on the political balance of power. Both seats already belonged to the Conservative Party, which has 22 members in the whites-only chamber of Parliament compared to the ruling National Party's 133.

But the Conservatives' margin of victory in both contests is likely to send a strong signal to President Pieter W. Botha that his party is in trouble in the rural platteland farming area and that a white backlash over proposed reforms and relaxation of apartheid could be decisive in future elections. Those include another by-election later this month in Randfontein and nationwide local elections in October.

Some political analysts said they are convinced that the Conservatives' strong showing will compel Botha to seek a postponement of general, whites-only parliamentary elections scheduled for next year. That would give him time to sell his reform program and make adjustments that would make change more palatable to conservative whites.

Calling yesterday's losses "temporary disappointments," Botha said today that national security would be a high priority, but that economic, social and constitutional reform would continue.

"But I wish to draw the attention of South Africans to the fact that excessive demands often lead to excessive reactions," the president said in a statement in Cape Town. He said "radical elements" had frightened "patriotic voters" and added, cryptically, "This will have to be rectified."

Antiapartheid activists expressed fears that Botha, who has grown less mindful of international opinion, will begin retrenching his reform moves, justifying the retreat by citing the Conservative Party gains.

In Standerton, whose voters are made up largely of farmers and mineworkers, the Conservatives tripled their plurality from last year, when they captured the seat from the Nationalists. Rosier de Ville defeated Nationalist Hennie Erasmus, 9,078 to 6,224, regaining the seat that he had to give up after last May's election because of a technicality.

In Schweizer-Reneke, where the Conservatives squeaked by last year with a margin of only 151 votes, Piet Mulder defeated the National Party candidate, Willie Lemmer, 6,400 to 5,606, a fivefold plurality increase. Mulder is the son of the late Cornelius P. (Connie) Mulder, a Cabinet minister who was disgraced in a 1978 scandal over secret funding of government propaganda.

The gains were made despite preelection moves by the government that virtually banned the country's leading antiapartheid groups.

In both by-elections, the Conservative Party was determined to show that its rise last year to become the official opposition in Parliament has not peaked and that it is capable of winning throughout vote-heavy Transvaal Province. The Conservatives hold four of the eight seats in the western Transvaal.

For its part, the National Party wanted desperately to blunt the growing Conservative swing and show that it could carry out limited apartheid reforms and still win rural votes.

The campaigns were heated -- and at times bitter -- with the Conservatives focusing on the recent integration of beaches in Durban, where many rural Transvaalers vacation, and on the erosion of compliance with the 1950 Group Areas Act, which segregates residential areas by race.

Also at issue were recent economic steps by the government, including wage restraints and other forms of fiscal discipline.