For the first time in the 13-year history of white-ruled Rhodesia, the country's whites are failing to rally behind their previously undisputed champion. Ian Smith, and are openly criticizing his lack of leadership at a time of major crisis.

Caught between a rising white backlash and an ever-hardening black guerrilla stance, the fast-aging, 59-year-old prime minister seems to be standing more alone than ever before in his storm-filed political life. While he is in no immediate danger of being forced to resign, his room for maneuver at home and abroad has suddenly narrowed enormously.

Ironically, for a man branded outside as a hardline white supremacist, it is Smith's relative moderation that is getting him into trouble in white Rhodesia these days.

Reaction among the 230,000 remaining whites to his speech Sunday announcing only limited new measures to deal with the widening guerrilla war has been deep disappointment and even anger. Smith declared martial law in some areas of the country and a crackdown on the black internal opposition but rejected total mobilization and nationwide military rule.

Following the shooting down of an Air Rhodesia passenger plane by guerrillas and the slaying of 10 of the 18 survivors, the mood of the white community is such that there are serious doubts Smith can win approval in his promised white referendum this fall on a new constitution that would bring black majority rule next year.

This means that his entire March 3 agreement with three moderate internal black leaders is in serious danger of being rejected by the very constituency that has followed him so loyally and even bindly ever since the whites declared their unilaterial independence from Britain 13 years ago this November.

There were several calls for Smith's resignation last week from ordinary whites. Far more seriously, a number of his own cabinet ministers have been campaigning openly against the March accord and are calling upon their constituents to vote "no" in the referendum on the new constitution.

Such indiscipline among leading members of the ruling white Rhodesian Front was virtually unheard of before and reflects the growing disenchantment within the party's all-powerful caucus with the course events was taking in the country.

At the same time, Smith's highly controversial bid to open secret negotiations with one of the guerrilla leaders, Joshua Nkomo, has met with a storm of criticism. Any future talks are certain to be vastly complicated by the white backlast against this black nationalist leader following his statement taking credit for downing the Air Rhodesia passenger plane.

Nkomo's warning in Lusaka yesterday that more passenger planes may be shot down seems certain to widen to divide between him and Rhodesian whites. Until a week ago whites here generally regarded Nkomo as a "moderate" and "reasonable" man who should be drawn into the present transitional government.

A very common reaction to Nkomo among whites these days is, "I would kill him if I could get my hands on him."

Conservative, middle-of-the-road and liberal whites all made statements yesterday highly critical of Smith's Sunday address. The only slightly favorable comment, strangely enough, emerged from the pro-Nkomo Zimbabwe Times.

The right-wing Rhodesian Action Party, now enjoying an upsurge of popularity, called Smith "politically and morally bankrupt" and demanded that he hold a new general white election.

"It was one of the weakest speeches I've ever heard," said Guy Larche, the party's national chairman. "It leaves Rhodesia exactly where it has always been under Smith - getting weaker and weaker."

He warned that Rhodesia would "drift into anarchy and possibly a bloodbath" unless Smith abandoned the March settlement, dissolved the transitional government and launched a fullscale war against the guerrillas.

Surprisingly, the main pro-government newspaper, the Herald, was in its own way equally critical of Smith's speech, terming it "a damp squib" compared to what had been expected in terms of the anticipated "new course" in government policy.

After the big buildup of the last few days, many Rhodesians felt Smith's speech was too full of generalities, and that there was too little of the "possitive and firm course" the government was supposed to have been planning.

As for the moderate National Uni-fying Force, it called Smith's address a "woolly ramble" and said there was "no clear policy either to save the failing March 3 agreement or to replace it."

The party called on Smith and his Rhodesian Front to resign and make way for a "national government" to negotiate an end to the war.

Only the black-owned Zimbabwe Times offered faint praise for Smith's show of "some restraint" and for his keeping the door open to an all-party conference.

The reaction of many ordinary white Rhodesians was no less hostile than the press and the various parties. The Herald reported it has been besieged by telephone calls from whites across the country "all expressing disappointment." It quoted one who said Smith had offered "no action for a nation waiting for action."

One white interviewed in the streets of the capital said he and most of his friends were "tremendously depressed" with the lack of direction coming from Smith and the transitional government.

Just where Smith is taking white Rhodesia remains something of a mystery, although the signs still point toward negotiations with the guerrillas even as he keeps the fiction of the failing transitional government alive.

If this proves true, then Smith is likely to face the biggest challenge of his political life from his own white followers in the near future - provided enough of them remain to see Rhodesia through to the new Zimbabwe.