Prime Minister Ian Smith, dubious that the latest initiative by Great Britain and the United States will bring a political settlement for Rhodesia, is ready to make a new long-shot try for a moderate black government intended to pose a test of sincerity in London and Washington.

In an interview at the prime minister's office, Smith could hardly have been colder about the Anglo-American effort, conducted by British Foreign Secretary David Owen. But he made clear the alternative was not a war of attrition where time is on the side of the guerrillas. "I do reassure you that I have got other thinking," Smith told us. "We have other irons in the fire."

To identify the irons now, the prime minister said, would be bad tactics." He did not rule out his own resignation and replacement by a black man but seemed far more inclined to seek the same goal in two steps - perhaps through elections. Smith is unquestionably in tune with South Africa's intent for Rhodesia quickly establish a black government that surely would be rejected by "front-line" black African states bordering Rhodesia but which could confront the West with difficult choices.

So, Ian Smith, having stubbornly staved off the whole world for 12 years running Rhodesia's white minority government, is about to play what may prove his last card. Although his long suit is doggedness rather than dexterity, how he plays his card may determine whether Rhodesia's belated changeover to black goverment will be orderly and Western-oriented or chaotic and Communist-aligned.

A signal that something new was up came June 15, when R.F. (Pik) Botha, South Africa's foreign minister, conferred in Salisbury with Smith and his cabinet for three hours. Before coming here, Botha was privately musing whether Washington and London could reject democratic elections on a broad-based black goverment put in place by Smith. Was this discussed here June 15? Smith would not tell us, but he did not deny it.

In our interview, Smith was less equivocal about the Anglo-American initiative. He predicted black Rhodesian leaders backed by guerrilla forces would reject elections but noted Owen's statement that this would not prevent settlement. Does Smith think this reversal of past British policy will stick? "One hopes he will abide by it," Smith said icily.

The reason for pessimism about the Anglo-American effort, Smith went on, is resistance to "the necessary safeguards in order to ensure the confidence of white Rhodesia." He may only be storing up bargaining chips by his proposal that voting be limited to "qualified" blacks - an idea unacceptable to everyone else concerned. But on maintaining the Rhodesian police and army and a trust fund providing financial guarantees to white Rhodesians, Smith is deadly serious.

Given the impasse, liberal white businessmen here, who criticize Smith for always reacting rather than initiating, fear he will commit Rhodesia to waging an unwinnable war. But the prime minister - while contending "people would be amazed at how long Rhodesia can go on, if we have to" - conceded he is "concerned" about white emigration, which is "having an effect on the economy and the miulitary effort."

Thus he declared, "Although his stepping down as prime minister is not "part of our thinking at the moment," he added. "If I was convinced that it would help, I want to assure you I would very ready to do this."

Smith needs black support much broader than he sought in earlier unsuccessful efforts at an "internal settlement." A sign that he understands this was Smith's statement to us that while guerillas are Communist-controlled, their political representatives inside Rhodesia are not. Here is a tiny opening for a black government that, while displeasing the "front-line" states, would test the West's real intentions.

Such a test would please Ian Smith, who is "disillusioned and disappointed" by "the Free World, of which he considers himself a part. "If you're a Communist you're given everything - on a platter, gratis," Smith told us. "If you're a member of the Free World in southern Africa, you gfet nothing from them even if you're prepared to pay for it." Angola, he said, showed that was so even for blacks vs. blacks.

But much more than white Rhodesian anger about ethnic brethren in Britain and the United States is at stake in the play of the prime minister's last card. It looms as the remaining possibility, long though the odds are, to save Rhodesia from economic and political anarchy leading to oppression at possible Communist control.