THE CONTEST for leadership of the Senate Democrats has broken out into the open: the actual choice won't be made until after the elections, but the crucial maneuvering is going on now.

Senate leadership positions are chosen by secret ballot, and typically the number of commitments claimed by the contenders exceeds the number of senators. So there's no way to know whether Robert Byrd, the incumbent minority leader and an expert vote-counter, has the votes that he says he does. But there is a questioning of his performance, whispered in the cloakrooms and voiced more audibly around town.

When Mr. Byrd beat Edward Kennedy and became whip in 1969, Democrats had a majority in a Senate that was deeply riven on the issues. At that time Senate Democrats wanted their leaders to schedule matters fairly and efficiently, keep members informed and tend to housekeeping. No one disputes that Mr. Byrd has done these things ably.

But since 1980 another kind of leadership has been seen on Capitol Hill. Republican leaders Howard Baker and Bob Dole have welded together a majority of disparate senators in support of a series of constructive programs. Tip O'Neill and other Democratic leaders have done something similar in the House. But Senate Democrats, though no longer irrevocably divided on such issues as civil rights and Vietnam, have nonetheless been unable to present a reasonably united, constructive front. This is what Bennett Johnston says he can provide, and he argues, at least by inference, that Mr. Byrd cannot.

Mr. Johnston has the right recipe, and it is up to Senate Democrats to decide who is the right cook. It has often been said that Mr. Byrd is a poor choice for leader because he is not a good performer on television. That misses the point. Over time voters know how to look past style and judge substance: look at Mr. O'Neill's high job rating. What the Senate Democrats need to prove is that they can put up a set of candidates as distinguished in their party as the Republican leaders are in theirs, and that they can define and engage the issues as a more or less responsible -- if not unanimous (after all, they're Democrats) -- group. That is what they ought to be thinking about when they choose their leader after November's election.