BELGRADE, Yugoslavia (AP) _ Premier Branko Mikulic, under pressure to resign for mismanaging the economy, got a temporary reprieve Wednesday when Parliament postponed debate on his disputed budget, an official report said. The premier's 1989 economic plans are seen as a possible threat to the survival of his government, if the budget debate rescheduled for January turns into a vote of confidence.

Debate was postponed after the northern republics of Slovenia and Croatia rejected a draft federal budget and demanded it be streamlined, the official Tanjug news agency said.

The Parliment's finance committee introduced ''emergency financing'' of the budget until a final agreement is reached, Tanjug said.

In another apparent boost to the beleaguered premier, leaders of the national trade union federation Wednesday refused to demand Mikulic's resignation, Belgrade radio reported. It said union leaders decided to withhold their judgement until they review the premier's performance in January.

In the past month, the government has come under increasing attack from the press, which blames Mikulic's economic policies for the 250 annual inflation rate, the 15 percent unemployment and a sharp drop in living standards.

The country this year registered more than 1,000 strikes by workers protesting their plummeting buying power.

On Tuesday, more than 1,200 railroad workers walked off their jobs in Slovenia to demand pay increases and the resignation of the premier. The strike caused major disruptions in rail traffic, and railway officials brought in engineers from other parts of the country to break the strike, according to media reports.

The railroad workers returned to their jobs Wednesday after they were promised that a government-appointed comission will look into their grievances.

The average salary of the striking engineers, who were demanding a 100 percent wage increase, is about $300, Tanjug said.

The average monthly salary in Yugoslavia has fallen from $110 dollars to $87 dollars this year, according to the state-run press.

In 1988, wages have dropped 8.8 percent in real terms and productivity by 2.1 percent, according to official statistics.

Oscar Kovac, a government minister in charge of negotiating the rescheduling of the country's $21 billion foreign debt, resigned earlier this month in protest of what he termed ''pressures'' on the government to abandon market-oriented policies.

Mikulic's reputation has been further dented by a corruption scandal involving hundreds of government officials in his native republic of Bosnia. The officials have been accused of obtaining illegal financing for building luxurious vacation homes. Mikulic has denied any involvement.