HAVING SOLEMNLY promised to hold elections for a black majority-rule government by Dec. 31, Rhodesia's biracial transition regime now says it can't do it until April. A need for more time to prepare the new constitution is cited. The deeper reality is that the general unraveling makes it unlikely that elections - or meaningful elections - will take place at all. To foes of Prime Minister Ian Smith and the black nationalists working with him in Salisbury, this simply confirms the bankruptcy of the "internal" settlement. Smith supporters can only be dismayed that elections, critical to earning Salisbury Western support, are fading into the miasma of the war.

The guerrilla cause, meanwhile, is not exactly prospering. Punishing raids on the sanctuaries by the Smith forces, the threat increasingly posed to Zambia's political fabric by the guerrillas it hosts, the boost given Salisbury by Zambia's opening of its border with Rhodesia for economic relief, growing differences between the two guerrilla factions and their sponsors among the front-line states, and the distraction of Uganda's invasion of Tanzania - all this has combined to dispirit the forces of the Patriotic Front and to reduce at least for a time their military effectiveness. This hardly means that the casualties of the war, and its numbing economic and social consequences, are at an end. It does seem to mean that, just as the Front has kept the internal people from consummating their goals, so the internal people have kept the Front from consummating its own.

It would be consoling to say that finally the two sides are ready to compromise. In fact, they are ready for, or resigned to, continued struggle. Neither side has an evident policy, a plan aimed at taking it to its desired goal. Nor is there an evident opening for the sort of diplomacy by which the United States and Britain have been trying to transform a warring Rhodesia into a peaceable Zimbabwe. The Anglo-American plan lies, if not in ruins, in limbo.

So it happens that the liveliest question being asked about American policy these days is no-longer what the United States might do, beyond remaining at the ready on the sidelines. The question is whether Washington should try to make a virtur of necessity - some would call it failure - and declare that its inactivity is purposedful, intended to make plain that the United States no longer accepts responsibility for the outcome. This is the sobering point to which events and its own acts have brought the Carter administration, which launced its southern African policy almost two years ago hopeful, if not confident, that the United States could steer the region towards peaceable change.