A BARE MAJORITY of Prime Minister Rabin's Labor party has now chosen him to lead the party into the Israeli elections in May. The delegates' lack of enthusiasm was understandable. In two and a half years as prime minister, Mr. Rabin has shown himself to be an earnest plodder without a talent for lifting people up or for devising imaginative policy openings. Given Israel's murderously difficult circumstances, could anyone have done better? Defense Minister Shimon Peres thought he could but the party delegates put him down. Inflation, mismanagement and corruption are expected to cut heavily into the Labor vote in May, but Mr. Rabin is still the favorite to win. He may have Yigal Yadin's new Democratic Party for Change (most of the change is unspecified) as a coalition partner. In any event, he will not have a noticeably stronger domestic base for diplomatic maneurvering than he has enjoyed in recent years. No one familiar with Israeli politics expected otherwise but it is not a cheering prospect all the same.

The Labor party, which has governed Israel for all of its 29 years, did make one potentially important departure in its platform. Israeli platforms, it should be noted, are binding. Previously, the platform committed Labor to call a general election before yielding any West Bank territory in a negotiation. The new one authorizes the leadership, if elected, to withdraw from West Bank territory without an election. This doesn't mean Mr. Rabin could deal back all the land there that the Arabs seek. Isaelis still insist that, in return for the limited no-contracts peace that Arabs currently offer, they will pull back just to "secure" frontiers. It is a step forward, nonetheless, that Labor is positioning itself for the negotiations expected later in 1977. It gives the United States something to carry to the Arabs to persuade them to move toward the full-contacts kind of peace Israel desires.

Mr. Rabin arrives in Washington next week, the first of the Mideast leaders invited by President Carter by way of his own preparations for a major peace effort. The Israeli leader knows that, not even for this value-oriented administration, he cannot simply bask in the glow shed by his country's latest demonstration of democratic vitality. He will be doing his darnedest to enlist the United States in a common negotiating strategy and, since such a strategy will not be always be forthcoming, in a pledge of close consultations. The United States, while it must give concrete grounds for the confidence the Israelis need to negotiate effectively must not give grounds so concrete that Israelis will not have incentive to negotiate. It is a familiar problem and it is in its solution that the toughest challenge to Mr. Rabin's leadership, and to Mr. Carter's Mideast diplomacy, lies.