The most serious internal political crisis in Rhodesia's 11 years of illegal independence threatens to cripple the government of Prime Minister Ian Smith, forcing the defiant leader to fight for his political survival as public criticism of him mounts.

The crisis, which has been building for months, climaxed over the weekend with the resignation of the ruling Rhodesian Front Party Chairman Des Frost, a powerful and popular figure, who blasted Smith for a "total lack of leadership, planning and direction."

This move follows the emotional resignation Friday member of Parliament and former Cabinet minister Wickus de Kock - the great-grandson of a noted white Rhodesian pioneer - who also announced that he was emigrating to South Agfrica. Two months ago, 12 members of Parliament and the deputy chairman of the party, Harold Coleman, were expelled from the Rhodesian Front after publicly criticizing the prime minister.

The series of resignations and expulsions is a devastating blow to the once fiercely united party, which has completely dominated Rhodesian politics for 14 years. Even more potentially dangerous is the threat the split in the party presents to current efforts to press Rhodesia toward majority rule.

The split within the party first developed over Smith's move to repeal some of the country's discriminatory legislaiton especially the Land Tenure Act, which gives half the land to the more than six million Rhodesian blacks, and the other half to 270,000 whites. The 12 expelled members of Pariament argued that the action went against party principles.

The sdissension grew to include opposition to the Anglo-American effort to find a peaceful way to establish majority rule and end the 11-year-old constitutional crisis and four-year-old guerilla war.

Frost has been at odds with Smith since late last year over the government's acceptance of black rule within two years. In his resignation statement. Frost declared: "I will not allow my country to be handed over."

There are increasing reports from Salisbury that are expected, and that a new conservative party will be formed to challenge Smith and the Rhodesian Front .

The losses already mean that Smith does not have the majority required to change the constitution. He needs 44 votes in Parliament, but now has only 37. This endangers Smith's chances of gaining enough support to pass settlement proposals.

Sources have indicated that Smith will be forced to resolve the crisis by calling a general election within the next two months, although there has been no official comment on that question.

The bitterness of the split became apparent last night when the Rhodesian leader said his party was "well rid" of Frost, whom he described as "completely two-faced." "This gives you some idea of the problem I have had for the past few years trying to work with him," Smith added.

The prime minister added that there are "at least a dozen more" key Rhodesian Front officials who "if they were honest" should have left the party by now.

T"The party will much healthier once this malignant growth has been completely excised," he said.

Smith is also facing new oposition from moderate whites, who over the weekend merged the few opposition parties into the National Unifying Force, led by Allan Savory.

The moderates stand for "a smooth transition to black majority rule" in the near future, a position that has gained signigicant suppport from several branches of Rhodesian commerce and industry.

The deteriorating political situation had led to new confusion and a dramatic decline in morale among the country's remaining whites. The emigration figures for Mary were tha highest since Rhidesia broke from Britain in 1965 - 1,754 whites left bringing the total for the first five months of 1977 to almost 7,300.

It was announced in Salisbury that more than 4,500 men of military age had left the country since the begining of 1976. This corresponds to the escalation of the guerrilla campaign. This is a particularly serious loss for the troubled southern African territory since it needs every sbaable male to combat the increasing guerilla incursions.

The exodus has become so serious that the prime minister has twice publicly pleaded with whites to at least "wait and see" the results of the Anglo-American settlement attempt, an unprecedented move for the proud leader who for 11 years has denied any crisis of confidence in his government.

Many whites now have little hope that the settlement effort will be successful, particularly in light of the militant positions of Rhodesia's black nationalist leaders and the five "front line" presidents of Zambia, Mozambique, Tanzania, Angola and Botswana.

Over the weekend, Robert Mugabe leader of the guerill's Patriotic Front, said that the Anglo-American proposals for establishing one-man one-vote rule were "not enough." He added that he opposed a peace-keeping force drawn for Commonwealth countries during the transition to majority rule.

The Anglo-American peace team, headed by British envoy John Graham, and U.S. ambassador to Zambia Stephen Low, was scheduled to arrive in Zambia today on the first leg of three weeks of critical talks with black and white leaders in southern Africa on a "package deal" worked out by British Foreign Secretary David Owen and U.S. Secretary of State Cyrus Vance. Owen is scheduled to tour southern Africa later this month to handle the final stages of negotiations.