Former Cabinet minister Connie Mulder was expelled from South Africa's ruling National Party today for refusing to accept the main responsibility for the covert use of millions of dollars to influence world opinion about this country.

Two separate government reports have stated that Mulder, Eschel Rhoodie, the top civil servant in the Information Department, and Hendrik van den Bergh, the nation's chief of intelligence, were solely responsible for the diversion of $73 million to the propaganda campaign.

Mulder has said that former prime minister John Vorster and the current finance minister were involved from the start in one of the campaign's more spectacular efforts, the funding of a pro-government paper. The Citizen.

Mulder's insistence that others, including Vorster, where involved is expected to hinder the government's attempts to dispel suspicion that it is engaged in a cover-up.

Prime Minister Pieter W. Botha's credibility is riding on public acceptance of the two official reports. National Party officials had issued an ultimatum to Mulder, who spent 28 years in public life as a party member, that he publicly accept the official accounts of the scandal or face expulsion. They ousted him today after he refused to back down.

Their action was the final political humiliation for Mulder who lost to Botha as the National Party's choice for the prime ministership by only 24 votes in last September's caucus.

Since the scandal began to unfold in May, Mulder has lost his posts as minister of information and of black affairs.He was also forced out of his high position in the National Party hierarchy and his seat in parliament.

Mulder in recent weeks has talked about running for his old parliamentary seat as an independent.

Meanwhile, the chances that Vorster, now South Africa's president, might be able to ride out the scandal appeared to grow dimmer. Two Afrikaans newspapers called this week for his resignation. CAPTION: Picture, CONNIE MULDER . . . rejects official version