YOU WOULD GUESS -- would you not -- that the Carter administration, devoted to majority rule in Africa, would be eager to observe the elections coming up April 20 in Rhodesia? Some number of the 3 million eligible black voters will decide whether to accept he particular form of majority rule endorsed by the 95,000-person white electorate in January. Observation would help the administration ensure that Rhodesia's blacks get the government they want. It would also let the administration demonstrate to congressional skeptics that it is prepared to support the "internal" settlement over the guerrillas, if that is the people's choice.

Moreover, the foreign-aid law of 1978 requires the president to determine, by way of deciding if the United States should lift sanctions, that: "A government has been installed, chosen by free elections in which all political and population groups have been allowed to participate freely, with observation by impartial, internationally recognized observers."

But -- no surprise -- the administration is not going to send observers. To do so, the argument runs, would be to be taken in by an Ian Smith trick to perpetuate the essentials of white rule under a black facade. This, the State Department fears, would render it suspect both as a mediator in Rhodesia and as a friend of freedom in black Africa overall. And if that provokes grumbles about it s evident reluctance to support the principle of elections, so be it.

Fortunately, others perceive the difference between observing an election and endorsing its results: There is a move now in Congress to send a corps of professional observers (not legislators). Monitoring a first-time-ever election in a large country partly under martial law and partly under guerrilla control is bound to be a complex and controversial undertaking, but it seems to us right and necessary. How can a democratic country not measure the democratic performance of another country asking to be measured? How can the United States set a fair-elections test on sanctions and not see how it is passed?

Congress should not kid itself into thinking that elections will somehow settle the Rhodesia problem, even if the elections are genuine and sanctions are then lifted. Ian Smith, the white leader, evidently counts on such a magical breakthrough. There will remain a terrible problem -- the war -- with which he and his black colleagues must still deal. But what the United States should do, and at this point perhaps all we can do, is to indicate that elections are the preferred American way.