SEN. JESSE HELMS'S Rhodesia amendment, which would lift sanctions and permit trade with the United States for six months, introduces Congress into policymaking on Rhodesia at a crucial juncture. An earlier version, calling for a 15-month suspension of sanctions, came within three votes of passage. Two considerations help explain why a proposal vehemently opposed by the administration did so well. First, the moderate, one-man, one-vote, multiracial system promised by the "internal" government that was set up last March looks better to many senators than the communist-oriented black guerrilla regime they see as a likely alternative. Second, perceiving the administration's policy as essentially pro-guerrilla, these senators thought that by temporarily lifting sanctions, they might even the odds in a struggle the guerrillas now seem almost sure to win.

Ian Smith and the internal black nationalists do promise a result compatible with American interests and values, or one more compatible than the Angolan-style regime that could emerge from the black civil war likely to follow the collapse of the internal regime. The protections for whites in the internal setup offend some Africans, including the "front line" states, and the State Department. We have long felt, however, that the constitution of Rhodesia/Zimbabwe should reflect the preferences not of the front line states or of the State Department but of its people. The "external" guerrillas are bound on blocking the elections that would legitimize the internal settlement. They insist they will not submit their own political fate to a fair, internationally supervised vote. Meanwhile, the United States and Britain urge compromise, but the internal people won't go along because they are losing, the external people because they are winning. The war goes on.

American policy is tipped toward the guerrillas. Notwithstanding Salisbury's adoption of majority-rule plans, the State Department treats Salisbury like a leper and courts the Popular Front. The United States funnels economic aid to the front-line states sponsoring the guerrillas, and enforces no-trade sanctions against Rhodesia. There are serious rationales for this bias: to please the front-line states and nations like Nigeria, and to preempt direct Cuban-Soviet military people, who feel, not without reason, that they are paying for an American policy conceived for larger ends.

But will the Helms amendment even things out? The argument for it is that it will bring guerrilla leader Joshua Nkomo, regarded by Salisbury as still a tolerable figure, into a negotiation that he rejects now because he thinks he can win it all in battle. The trouble with that argument is that it skips over the possibility that Mr. Nkomo, rather than negotiating, may instead call in the Cubans. To that the response is that the internal people are prepared to fight. But could the United States then stand by?

It is late in the day to try to even the odds in Rhodesia in this way. The situation seems to us to have deteriorated too far. For all the flaws in administration policy, the Helms alternative promises no more than messy confusion. Americans who respect the internal effort might better press the administration to induce more moderation from the Patriotic Front. Why not put the front on notice that American good will depends on its delivery of a reasonable degree of democratic procedure and racial fairness? The "signal" the administration now sends the front is: almost anything goes, if you don't call the Cubans. That is what dismays Rhodesian and American critics alike.