DURING HIS LONG campaign for the presidency, Jimmy Carter promised to select new federal judges and U.S. attorneys solely on the basis of merit. It seemed to be a foolish promise; the president simply lacked power to make good on it. In the real world of politics, the Senate had controlled that selection process for decades and seemed unlikely to give up its power. But President Carter and Attorney General Griffin Bell have won over so many senators to merit selection that there is now a chance in the next few months to demonstrate its worth.

The president now has 166 federal judgeships to fill - 152 new ones created by Congress last month and 14 vacancies. That's one new judge for every three now on the bench. More than half of those 166 new judges are to come from lists submitted to the president by nominating commissions. These include the 39 new appellate judges and about 60 district judges in 19 states whose senators have agreed to relinquish part of the nominating power they have held for so long.

Under the new system, the administration appoints the nominating commissions for the appellate judges and the senators of each state are asked to form similar commissions for district judges. Each commission reviews all candidates for the jobs and submits to the president the names of three to five individuals it believes are well qualified. The president then makes a final selection from that list. This is a sharp departure from the past, when the president was told by the senators whom to nominate and usually did as he was told.

Whether or not these commissions will produce better judges than did the old patronage system remains to be seen. Commissions can be loaded with Democrats, for example - or Republicans. Or commissions can yield to powerful politicians. Indeed, the only judicial nomination by President Carter to encounter serious opposition was a nomination made from a commission's list. That was his selection of an Iowa man who had been one of his staunch supporters but was regarded by the American Bar Association and some Iowa newspapers as unqualified to be a judge.

The administration is trying to get around problems like these by asking that the commissions be broadly balanced and that the selection process be open. Attorney General Bell has said he will make public the names of those recommended for the judgeships as well as the names of those who are on the commission. By doing that, he is giving the public - for the first time - a chance to comment on potential judges before the president and the senators involved have made a final choice.

If President Carter and Attorney General Bell can make this plan work, other senators will be under strong pressure to join in the plan, and the president will have made good on an important campaign promise.