In the controversy about whether to life economic sanctions against the new biracial government here, a key question is the fairness of the constitution that was approved by white voters in January.

Critics say that the constitution does nothing to change the racial situation here, where for nine decades a small white population ruled the native black majority. They charge that the document effectively guarantees white control of the government for years to come.

Backers of the new government counter that the constitution represents the most that blacks could possibly hope for at this point and that true majority rule "is here," in the words of the newly-elected black Prime Minister Abel Muzorewa.

Most controversial are provisions of the constitution under which whites are reserved 28 seats in the 100-member Assembly and one-third of the Senate seats, despite representing only 4 percent of the population.

In addition, whites retain control of the top military and police posts and high court judgeships. The civil service also remains under white control, and entrance requirements make it difficult for most blacks to qualify for government jobs. Any changes in the bureaucratic structure, including the replacement of present government functionaries, must be approved by the white Civil Service Commission.

Secretary of State Cyrus Vance cited such sections of the constitution to explain President Carter's decision not to lift sanctions against the former British colony.

"This constitution emasculates the man running the country," said one constitutional analyst. "No self-respecting black leader would accept it."

Yet another analyst, a black, argued that "this constitution has finished the monopolistic power of the Rhodesian Front," the party of former prime minister Ian Smith. "There is a hole in the dam. We don't know how big it is, some whites will try to plug it and the blacks will try to enlarge it, but the fact of the matter is that there's a hole in the dam."

Nevertheless, the white minority here holds that amounts to a veto over fundamental changes in the government. Their leaders argue that continued white participation in the government is essential to prevent chaos.

Central to the constitution are provisions guaranteeing white control of the military, which is waging an escalating war against an estimated 12,000 nationalist guerrillas inside the country and many more being trained in camps in nearby nations.

The principal architect of the constitution, Solicitor General George Smith, has reacted sharply to charges that the new biracial government serves as a "puppet" for the white minority. Those charges, he declared, "indicate a complete bias and are deliberate distortions of our constitution."

Referring to criticism that the white third of Parliament can block changes desired by the black majority, he said, "Government is run by the passing of legislation. Over 300 laws of the country can be amended by a simple majority of Parliament."

Under the constitution, Prime Minister Muzorewa also holds the title of minister of defense and combined operations, heading the agency that directs the war against Patriotic Front guerrillas.

According to the letter of the constitution, combined operations chief Lt. Gen. Peter Walls and Army commander Alexander MacLean "shall comply" with directions they get from Muzorewa on the conduct of the war.

In the hypothetical situation of a disagreement between Walls and Muzorewa or the refusal of Walls to comply with a decision by the prime Minister, Muzorewa can constitutionally abolish Wall's job, which exists only at the prime minister's discretion. MacLean's job cannot be ended by the prime minister.

Muzorewa, however, cannot dismiss Walls or reduce his rank arbitrarily. For that, he needs the approval of the defense forces commission. Its membership is restricted to whites for the time being because of the requirement that members have held a rank above colonel for at least five years. The highest rank now held by a black is lieutenant.

In practice, political realities would also enter into the resolution of any potential conflict between Muzorewa and his white defense chiefs. In the case of a serious disagreements, "Walls would have to decide whether to resign or stage a coup," said one political analyst.

"In political terms, whites are constrained because if they stage a coup they have so much to lose. If the world doesn't give recognition to Muzorewa, do you think they will give it to whites after a coup?" he asked.

Likewise, Muzorewa would have to consider the effect on white morale off an open clash with Walls. It is a basic political fact that the white-led armed forces are keeping 12,000 guerrillas from toppling his new government.

"In practice, Muzorewa is dependent for his survival on three things - an efficient defense force, an efficient administrative force and white morale, so there's no way he can argue against a decision of the military," one constitutional analyst observed. Others have noted that the prime minister has no way of double-checking on the intelligence Walls and MacLean choose to present him on the basis for military decisions.

The fact is that where the war is concerned, Muzorewa's interests and those of the whites coincide, which he made clear in an interview shortly after he took office.

"If Walls came here to say he was going to send people to hunt everybody in Mozambique, I would say 'no' and I know he would not do it. But if Walls came and said we have got five or whatever number of armed [men] and intelligence says on a certain day they are knocking us over, why would I say don't go and preempt that situation? Whyy on earth would I say that?" Muzorewa told his interviewer.

Blacks participating in the new government recognize the need to amend the constitution, although they are not prepared to say so on the record. Any amendments, however, require the votes of at least six whites out of the 28 in that national assembly. It is unlikely those votes could be mustered unless whites saw it in their best political interest. The refusal of the outside world to recognize the present government and life sanctions no doubt increases the chances the whites may eventually see the need for amendments.

At present, however, white officials scoff at the notion that altering the constitution would budget the West to recognize the new government.

"The West would not be satisfied [with changes]," said Smith. "Changing it is not going to bring [guerrilla leaders Joshua] Nkomo and [Robert] Mugabe back . . . can you expect the [guerrilla leaders] who have been fighting for control of this country for so long to come back and accept the position of an opposition or backbender?"

Some political observers speculate that the political realities of the new black-white political partnership and the need to make concessions to the black electorate will force changes in the constitutional structure to make it more acceptable to blacks.

"The constitution certainly does constrain black power. The question is will the whites want to constrain it so much they will commit suicide?" one non-white political science professor asked.