SECRETARY OF STATE Cyrus Vance has apparently failed in his efforts to meld the black guerrillas based outside Rhodesia and the multiracial administration governing inside Rhodesia. The external guerrillas repaid his courting by stiffening their demands for exclusive power, meanwhile, they are stepping up attacks on Salisbury. The internal group could and did, at no cost, pronounce itself ready for talks, but meanwhile it is pursuing its own plan to establish an elected one-man/one-vote government by the end of the year. The American hope to have each side sobered by the other is ebbing. The new hope is to arrange, or wish into being, a deal between a key moderate internal black (Bishop Muzorewa) and the more moderate of the external blacks (Joshua Nkomo). The alternative, the administration fears, is a wider war and perhaps Cuban-Soviet intervention, too.

The administration's anxieties are legitimate. What needs to be noted, though, is that public support is shrinking for its attempt to find middle ground. Officials find it good and necessary to continue an initiative appreciated by other African states. But, not without reason, it is provoking increasing puzzlement and challenge. What is so objectionable, for instance, about the internal people's establishment of a black majority government? If it had come at virtually any earlier point in the last 12 years, it would have elated Washington and London alike. Why is the United States so eager to offer a share of power in Salisbury to a fellow like Robert Mugabe, a self-proclaimed Marxist who believes that multiparty government is a "luxury?" The United States does not have to encourage Salisbury's policy of denying a political role to the guerrillas, but why should it convey an impression of understanding for the guerrillas' policy of refusing to participate in a political framework they do not control?

The possibilities of Rhodesian compromise may not yet be demonstrably so hopeless that the administration has no choice but to indicate on which side it believes the ultimate American interest lies, or to retire from the field. But there is substantial risk for the administration in maintaining what appears to be such a permissive and indulgent attitude toward the guerrillas. This can hardly strengthen its hand in negotiations - and is unquestionably costing it public confidence at home.