Jimmy Carter's wavering, waffling response to the Iranian hostage situation and the Soviet invasion of Afghanistan has drawn increasing fire from his frustrated opponents for the presidency. At first cowed by the possibility that criticism would sound unpatriotic in a crisis, they nevertheless are coming to realize that what was first seen as admirable restraint by the president may in fact be the symptom of a basic flaw in Carter's handling of foreign affairs. His policy has always reflected his pusillanimous tendencies when dealing with hard-nosed street fighters on the international scene.

Unfortunately, the president's hesitancy in earlier, less drastic situations was not overlooked by Ayatollah Ruhollah Khomeini or the tough, practical politicians in the Kremlin, and they decided that he was someone who could be pushed around.

Just such an opportunity to watch Carter in action -- or, more accurately, in inaction -- occurred last year in Rhodesia. The president was offered a no-nonsense policy to pursue in that volatile situation, but chose instead to do what amounted to nothing.

Feisty British Prime Minister Margaret Thatcher, stepping in firmly where Carter feared to tread, was able to bring the white-supremacist forces of Ian Smith and the black Rhodesian guerrillas to the conference table and hammer out a negotiated settlement.

Though the White House was not reluctant to accept credit for the success of the British-engineered settlement, secret State Department cables make clear that Carter contributed virtually nothing. Indeed, the administration's refusal to play hardball in a tough international situation made the British efforts even more difficult.

The unwelcome suggestion that Carter play a little practical politics to bring the Rhodesian imbroglio to a peaceful solution came, my sources tell me, from former Rep. Allard Lowenstein, an old Africa hand and former New York congressman who served at the United Nations with Andrew Young.

Almost exactly a year ago, Lowenstein proposed a plan that was roughly the same as the successful one used by the British months later. But a Capitol Hill source told my associate Bob Sherman that Lowenstein's plan was disregarded at the White House, the State Department and the National Security Council.

What Lowenstein suggested -- and the Carter administration thinkers apparently gagged at -- was that the United States use the economic and military sanctions imposed on Rhodesia years before as a means to get both sides to the negotiating table. The black guerrillas would be impressed by a threat to lift the sanctions, which would give the white Rhodesians power to keep control of the country indefinitely. Conversely, the white Rhodesians would be impressed by a threat to continue the sanctions indefinitely.

Playing both ends against the middle is not the Carter way of doing business, at least in international affairs. There is evidence that Carter's politically oriented chief of staff, Hamilton Jordan, may have failed to push this suggestion that a peaceful solution be achieved by knocking the two sides' heads together.

In an "eyes only" memo to the president Lowenstein said the possibility of lifting -- or maintaining -- the sanctions "can influence both sides to negotiate in good faith." But the memo was routed through Jordan, and it is not known whether the president ever saw it. What is known is that Carter never acted on the Lowenstein suggestion.

This was no will-o'-the-wisp theory cooked up by a pipe-smoking egghead far removed from the scene. Lowenstein had made an on-the-spot visit to Southern Africa last February to sound out leaders on both sides.

Not only did he clear his "big stick" sanctions policy with President Kenneth Kaunda of Zambia, but he was told by Ian Smith that Smith had "no problem" with renegotiation of the whites-only constitution, its submission to the entire nation -- blacks and whites -- and elections under U.N. supervision in which the black guerrillas would participate.

But the U.S. embassy in South Africa denigrated Lowenstein's diplomatic efforts, professing in a cable to Washington that Smith's reasonable attitude "boggles the mind." The embassy experts suggested that "Smith [intentionally] and Lowenstein [unintentionally] were . . . talking past each other."

So the administration sat on its hands. Eventually, the British moved along the lines recommonded by Lowenstein and brought the whites and blacks to the conference table.

But Jimmy Carter's failure to act was not lost on the cold-eyed obervers in Tehran and Moscow.