27 Quorn Ave. in the prosperious Salisbury suburb of Mount Pleasant is a roomy, red-tiled bungalow set back from the road amid two acres of garden. It used to be the home of a white family prominent in local racing circles.

Now 27 Quorn Ave. has changed hands. A stone wall has been built across the front lawn and the barred iron gate in it is guarded by a policeman with a rifle. Visitors are vetted by other guards in civilian dress. And, to their irritation, local residents walking their dogs are directed to the pavement on the far side. Since Feb. 1, Number 27 has been the home of the man his supporters are already calling "President Mugabe."

To his white neighbors, however, Mugabe remains "The Chief Terr" -- the terrorist leader who barely a month ago, was Public Enemy Number One. As cars pass his house now, their white drivers tend to give a prolonged and derisive hoot. Despite the security apparatus, a grenade was tossed at the house a few days ago; it exploded harmlessly against the new wall.

The arrival of Robert Mugabe, in other words, has brought to Quorn Avenue tangible signs of the tension and fear that pervade Rhodesia as its war-weary, embittered inhabitants count down to this week's decisive elections. wOnly now there is an additional fear: that the elections are going wrong; that the great gamble of Lancaster House is failing.

The gamble that Lord Carrington, the British foreign secretary, took at the peace conference in London's Lancaster House was that within the limited period he was willing to let Britain take precarious control of Rhodesia -- no more than three months -- it might be possible to implement a cease-fire, hold a free election and finally settle the Rhodesian problem with some semblance of order, even honor.

Part of that gamble has already come off; the cease-fire has taken effect more successfully than even Carrington dared hope or the Rhodesian intelligence community predicted. But as the end of Britain's temporary colonial rule under Lord Soames approaches, the plan is on the brink of falling apart.

For so massive has been the scale of intimidation during the election campaign that Soames' own election advisers now consider that the chances of holding a free and fair poll are remote. Almost certainly, therefore, Britain is about to preside over an election whose outcome will be fiercely disputed. Depending on the actual result and what political alliances might subsequently be arranged to form the first government of Zimbabwe, Britain could be faced with an explosion of guerrilla violence in the rural areas or a white-led military coup, just as Soames prepares to leave.

Just possibly, neither may occur. All that is certain is that the main responsibility for what is happening lies with the new resident of 27 Quorn Ave. And the dilemma that confronts Soames is that, if he is to hold the country together in the face of these increasing fears, he will have to rely more heavily than ever upon the existing, overwhelmingly white Rhodesian establishment.

But this in turn will reinforce the suspicions of Mugabe and his backers -- the most vocal being President Julius Nyerere of Tanzania -- that the British are just as intent upon eliminating his party and his revolution as the white Rhodesian establishment has been. What has gone wrong?

The beginning went well. Despite warnings from the Rhodesian military that the 1,300 troops of the Commonwealth monitoring force would be ambushed on their way to guerrilla assembly locations and rendezvous points or be annihilated once they were there, no serious incidents occurred. By the end of the seven-day period allowed them, more than 17,000 guerrillas had turned up from the bush. (Hundreds more were accepted later under an amnesty.) By the end of January, the camps held around 22,000.

Particularly encouraging for the British was the response in western Rhodesia of Joshua Nkomo's Zipra forces. They have always been better trained and disciplined than Mugabe's guerrillas; their command structure is more effective too. Nkomo's two principal guerrilla commanders, Dumiso Dabengwa and Lookout Masuku, were also helpful in dealing with incidents which arose in their camps. Only 400 of the 6,000 Nkomo guerrillas actually inside Rhodesia have failed to report to the camps -- and these Dabengwa regards as bandits who will be punished once they are captured. (Another 6,000 or so of Nkomo's troops -- many of them with advanced Soviet training as, for example, artillery specialists and even Mig23 pilots -- remain in Zambia, taking no part in the election.)

Nkomo, too, has been active in getting his forces to cooperte with the British. Persistently he has told them to stay in the camps so that free elections can be held. In one radio message read for him by Dabengwa, Nkomo declared: "The war is over. People must not be surrounded by armed men from any quarter." One result has been that Rhodesian police have already begun to train Zipra guerrillas to maintain law and order around their assembly camps and to provide a new monitoring system when the Commonwealth troops eventually leave.

What breaches there have been of the cease-fire have come largely from Mugabe's Zanla forces. And the disturbing evidence is that this has been a matter of policy. Between Dec. 22 and Jan. 6, more than 3,000 Zanla guerrillas infiltrated from Mozambique into eastern Rhodesia in defiance of cease-fire terms. Many of the arms they carried were hidden in caches inside Rhodesia. (The progress of these bands was watched by the Rhodesian combat tracker unit, the Selous Scouts, but the security forces were restrained by Soames from taking action.) Many of those guerrillas then made their way to Zanla assembly camps -- but many did not.

The figures show the outcome. Since the first meeting of the independent Cease-fire Commission early in January, 243 incidents have been reported -- from which the commission has winnowed 175 confirmed violations of the cease-fire.

Mugabe's Zanla forces have committed 83 of them, while 32 breaches occurred in areas of Zanla operation and were thus probably, but not provably, Zanla's responsibility.

Nkomo's Zipra has been responsible for 21 breaches; another nine have been in Zipra's territory.

The security forces -- which include the auxiliaries of the former prime minister, Bishop Abel Muzorewa -- have been responsible for two violations and 12 cases of incitement to violations. (Of the remainder, 15 breaches were by bandits armed with eastern-bloc weapons; 13 cases were unattributable.)

From their own, separate calculations, the British officials around Soames reckon that 85 percent of all the incidents in the country can be attributed to Magabe's Zanla forces.

What really threatens the chance of holding a free election is not this violence, however, but the intimidation which has gone with it.

All the major black parties in the election -- Muzorewa's United African National Council, Nkomo's Patriotic Front, Mugabe's Zimbabwe African National Union (Patriotic Front) and Ndabaninge Sithole's Zimbabwe African National Union -- are guilty in varying degress of intimidatory practices. But the evidence against Mugabe's ZANU officials and his Zanla guerrilla forces is overwhelming. It comes not merely from the Rhodesian security forces and Muzorewa -- that could be expected -- but also from his former partner in the Patriotic Front guerrilla alliance, Nkomo, and, more important still, from Soames' own election supervisors in the field -- the British local government officials and former colonial administrators flown in for the campaign.

Their conclusion is that the scale of the intimidation by Mugabe's supporters has so distorted the pattern of political loyalties that nobody can now know what the result of a fair election might have been.

This, too, is deliberate policy on Mugabe's part. The evidence has come from 300 Zanla guerrillas captured by security forces since the cease-fire. Among them were senior officers who have been interrogated by Soames' 19 British police advisers. In formal depositions, they confirm that a large proportion of Zanla forces in Rhodesia -- as many as 4,000 men, or more than one-quarter of the army -- were instructed to remain outside the assembly camps to exert pressure on the population to vote for Mugabe's party and to prevent other campaigning. They are now spread among the villages in groups of two or three.

That decision, according to the depositions, was conveyed by Mugabe's senior guerrilla commander, Rex Nhongo, before the ceasefire came into effect. With it came a warning from Nhongo that any subsequent counter-order instructing guerrillas to make for assembly camps should be ignored if it appeared to have been made under duress. Only an order given to them personally by Nhongo or his repersentative was to be carried out. (At Soames' insistence, Nhongo recently completed a series of radio broadcasts to his forces still in the bush telling them to report to assembly camps. Unsurprisingly, in view of those secret orders, the response has been nil.)

Not that things are much better around the Zanla camps. Zanla guerrillas and the youth who suport them have been moving in and out of their assembly camps to "politicize" the population around them. "Essentially," one senior British official said, Mugabe's "ZANU (PF) is waging a paramilitary campaign in this election."

The mere presence of Mugabe's guerrillas in villages in eastern Rhodesia is enough to deter the local population from showing support for any party other than ZANU. A stick grenade carried by a supporter or straight threats of death are strong deterrents. Equally effective, though, are the rumors that ZANU supporters have spread about their ability to detect which party a tribesman votes for. One rumor is that ZANU has black boxes which enable party officials to see into ballot boxes in polling stations; a similar story involves Russian satellites in the sky above Rhodesia.

Posters portraying Muzorewa are found with his eyes cut out -- a chilling reminder. In Salisbury, black servants have been visited by ZANU political agents and told their names will be entered into a black book. In rural areas, lists of village headmen have been ostentatiously compiled, inviting fears that these will be used as "death lists" if villagers do not vote en masse for Mugabe. Zanla forces have even taken advantage of the withdrawal of Nkomo's Zipra guerrillas into assembly camps in western Rhodesia to move into areas once controlled by Zipra.

Where even these pressures seem to be failing, murder is the ultimate persuader. Most days, the communiques issued from Combined Operations Headquarters in Salisbury list at least one gruesome political murder. "Exemplary murder has become something of a fine art," said one British official.

Earlier this month, to take one of the less blood-curdling examples, three Nkomo activists -- a candidate and two party workers -- were putting up posters in Chibi tribal trust land in south-central Rhodesia. They were seized by two gunmen, who identified themselves as Zanla men, adding that they had instructions from Mugabe to kill anyone who defied their ban on other parties in the area.

The three were then marched to two nearby villages where the population, clearly frightened, was assembled. The villagers were told to ignore Nkomo's party, informed that ZANU (PF) had equipment to detect how they voted, and warned that anyone who voted for other than Mugabe would have their heads cut off.

The two Nkomo party officials were then beaten up but managed to escape. The luckless candidate was last seen having red-hot coals stuffed down his throat.

The overall effect of such tactics has been dramatic. The conclusion of Soames' election supervisors is that in five of Rhodesia's eight electoral provinces, conditions for a free election no longer exist. Mass intimidation is recorded in Manicaland, Victoria, Mashonaland East and Central, with political thuggery in parts of Matabeleland South. Only Mashonaland West, Matebeleland North and Midlands are free of fear, according to the supervisors. In Victoria Province, the worst affected area, both Muzorewa and Nkomo have abandoned attempts to hold rallies. At a recent meeting in Salisbury, Nkomo said: "The word intimidation is mild. People are being terrorized. It is terror."

It is Muzorewa's party, however, which has borne that brunt of Zanla activities. Muzorewa is competing directly with Mugabe for the Shona vote, three-quarters of the electorate. Two weeks ago, Muzorewa claimed that since the start of his campaign on Jan. 25, he had addressed only one meeting where he could say there was no intimidation. He has urged Soames to exercise his powers to ban ZANU from the elections. hWhat is perhaps more surprising is that, in a series of private meetings with Soames, Nkomo has made exactly the same demand.

The reality is that Soames can do very little. He has issued a series of threats against ZANU in an attempt to get Mugabe to abide more closely by the Lancaster House agreement.

He has suspended the campaign of one particularly belligerent ZANU candidate, and banned ZANU meetings in the Chiredzi area in southeast Rhodesia where no other party has been able to campaign. He has taken special powers to disqualify parties from contesting a particular district if violence and intimidation continue. He can even disenfranchise entire areas if intimidation and violence are so high that free and fair elections are impossible -- a move questioned even by the conservative Salisbury newspaper, The Herald. And to combat ZANU's rumored "black box" spy, he has asked for 500 British bobbies to stand, unarmed, at the polling station.

But he cannot ban ZANU from the election. Mugabe has threatened to resume the war if Soames does use his power to ban ZANU from contesting any electoral district. And while there is room to doubt how effectively his guerrillas could now resume full-scale operations, Mugabe could certainly unleash violence far more savage than any Rhodesia is currently enduring.

Mugabe, of course, counterattacks with a catalogue of accusations of bias on Soames' part and a litany of atrocities allegedly carried out by Muzorewa's men, the 16,500 auxiliaries now operating as part of the security forces. As a result, the Commonwealth Monitoring Force has spend most of the last month investigating the auxiliaries' activities.

Their conclusions do not wholly exonerate them. There was a period of 10 days in January, after guerrillas had moved into the assembly camps, when the auxiliaries, deployed on the authority of Soames to help contain the wave of lawlessness which followed the withdrawal of the security forces from the rural areas, started to go out of control. Many of the accusations now made derive from that time.

At Soames' behest, however, the security forces commander issued draconian orders to rein in the auxiliaries. According to members of the Commonwealth Monitoring Group, those orders have largely been effective. According to Brigadier Adam Gurdon, the British chief of staff of the monitoring group: "From our own observations we found that 90 percent of the allegations running down the auxiliaries have been contrived for purely political reasons." At least two brutal murders attributed to the auxiliaries have turned out, on investigation, to be the work of Mugabe's men.

As for the allegations of bias, they are mostly trivial by comparison with the activities of Mugabe's forces. But it is of course true that Soames has to work through the existing Rhodesian administration. He has also made clear that he would confine his activities to implementing the cease-fire and preparing for free elections. Other issues -- the dismantling of the country's fearsome security laws, for instance -- he is explicitly leaving to the new government to decide.

Who that new government will be and what it will decide hinges not simply upon the election but upon the credibility of that vote, and upon the willingness of people to accept its verdict. But the question that now looms over the whole process is whether any new government will be able to decide anything before new violence erupts.

The truth is that Mugabe's efforts have made the prospect of such violence even greater than it was.