MR. CARTER TOOK a sizeable and perhaps unnecessary risk in deciding, barely a day before the British Foreign Secretary arrived in Washington, to identify himself personally with the quest for a negotiated settlement in Rhodesia. Presumably the President decided to receive Dr. David Owen, at a White House meeting that photographers were invited to memorialize, to demonstrate his commitment to black liberation. But it is not readily understandable why this needs to be demonstrated right at this moment, when the situation in Rhodesia could hardly be more precarious and the odds on the President's being able to influence events constructively could hardly be less promising; it is not, after all, as if the President and his designated "point man" for Africa, Ambassadr Andrew Young, had not been trumpeting their commitment to black-majority rule on the continent. In the meantime, diplomats far more familiar with Africa than either the President or Mr. Young have been - and still are, for that matter - at work on the problem. The situation on the ground in Rhodesia is bad and disintegrating. While it is unclear what the President can add, what he can lose is all too obvious. It seems a curious way to squander the most precious of assets in American government - presidential attention and prestige.

The weekend discussions appear to have done more than put Mr. Carter's weight onto the diplomatic scales. The talks evidently marked an end to the previously policy of searching for common ground between Rhodesian whites and their various black challengers, and the onset of a new policy of drafting a British-American plan - to be revealed and implemented in fairly short order - meant to produce a constitutional settlement.

The premise of the plan is that it should satisfy the contending forces in Rhodesia/Zimbabwe, if - a big if far it has not proved possible to fit together the blacks' demand for power (some want it by the gun, others by the ballot box) and the whites' demand for physical and economic guarantees under black majority rule. In any event, it is expected that world opinion will recognize the fairness of the new plan and will help gain a hearing for it on both sides of the line. Any resemblance of this approach to that taken by President Carter in the Mideast is, we gather, hardly coincidential.

Whether the guerillas, and the nationalist politicians trying to ride them to power, will slow down on account of the new Carter, initiative is uncertain. They will have to place a higher value than they now appear to on the preservation of the economic plant and the prospects for harmony and progress in their own country. But unquestionably the harder choice falls on Ian Smith, who has called limited-franchise elections for Aug. 31 to beat back a challenge from the right within his own party. If he fails to win a mandate, it's almost certainly all over for the whites granted, not a big one - to give a lease on life to a white community that, for all its mistakes and weaknesses, still has a contribution to make to the new Zimbabwe.