In a movie that could have far-reaching consequences for the new government of Zimbabwe-Rhodesia, a senior member of Prime Minister Abel Muzorewa's party announced today he was forming his own party and that at least eight members of the National Assembly were joining him.

The action robs Muzorewa of his slim majority in the assembly, although there was no immediate prospect that the government would fall.

The announcement was made by James Chikerema, a former senior vice president of Muzorewa's party who has a long background in nationalist politices. He is a former ally of Joshua Nkomo, coleader of the Patriotic Front guerrillas opposing Muzorewa's government, and he maintains a personal relationship with Nkomo.

"The guerillas may talk to these guys (in the new party) more easily than to the bishop (Muzorewa)," remarked one white political analyst.

A spokesman for the new party said it would "go to the corners of the earth to get peace in this country." Referring to Muzorewa's United African National Council, the spokesman added that "those who have failed to unite a political party cannot succeed to unite a nation."

The new party said it would demand seats in the Muzorewa Cabinet based on its strength in Parliament.

The rivalry between Muzorewa and the forceful Chikerema is rooted both in their personalities and in their different tribal affiliations. Most of the deputies who defected with Chikerema are, like him, from the Zezuru subgroup of the Shona tribe, which outnumbers the Manyika group to which Muzorewa belongs.

In August, Chikerema tried unsuccessfully to oust Muzorewa from the party leadership. Reports at the time said he was backed by major Western financial interests operating in Africa who hoped that his ties to Nkomo might lead to a political settlement of the raging guerrilla war.

Muzorewa failed to give Chikerema a post in his first Cabinet announced three weeks ago, a move that disappointed many blacks who regard the 54-year-old leader as a long-time fighter for black rights in this contry.

Chikerema was allied with Nkomo during the 1960s, but after a feud he formed his own organization, called Frolizi. In the early 1970s, Frolizi briefly engaged in guerrilla warfare, but it was considered ineffectual.

Chikerema joined Muzorewa's party in 1975 and wasa co-minister in the biracial transitional government last year that preceded the establishment of the new black-led government.

The emergence of the new party, called the Zimbabwe Democratic Party, is likely to complicate Muzorewa's effort to establish reforms as he campaigns for international acceptance of his government.

"The timing is totally wrong," said one source close to Muzorewa's party. "It'll make Congress uneasy as hell." The U.S. Congress is debating whether to lift economic sanctions against the Muzorewa government.

"At a time when people are looking for signs of strength, it will give the government a bad image," said one party member.

Officials of Muzorewa's party shrugged off the defection as destined to fail. But the only other black opposition group in the country, the Zimbabwe African National Union (ZANU) led by Rev. Ndabaningi Sizorewa government.

"We can hardly believe it can last two months," said ZANU spokesman James Dzvova.

The breakaway reduced Muzorewa's support in the 100-seat assembly, or lower house, from 51 to 43. The party line-up in Parliament when it convenes for the first time on Tuesday will be: Muzorewa's party, 43; Rhodesian Front (led by former Prime Minister Ian Smith), 28; United National Federal Party, 9; and the new Chickerema party, 6. Sithole has said his party will not accept the 12 seats to which it is entitled, charging that the April elections were rigged. CAPTION: Picture, Muzorewa, shown here celebrating his election victory in Salisbury, was shaken by the defection of several key supporters, who will form a new political party. AP