BY RESIGNING from the Senate yesterday, Harrison Williams of New Jersey spared his colleagues the difficult but unavoidable task of expelling him for his "ethically repugnant conduct" in the Abscam affair. His resignation preserved some measure of dignity and kept the senator's name from the short and ignominious list of 15 who have been voted out of the Senate by their own colleagues.

During his 28 years in Congress, Sen. Williams made some significant contributions. He was the first Democrat ever re-elected to the Senate from the state of New Jersey. He was a leader in the area of labor law, and some of the most important legislation protecting working men and women passed in the last three decades bears his mark. He was a strong and early supporter of civil rights laws and of the poverty programs of the '60s. His advocacy of alcoholism treatment and rehabilitation programs grew out of his personal experience with the problem and was all the more effective because of his own triumph over this illness.

These and other achievements over a long career of public service will be remembered by those who shared his goals and those whose lives were made better by his work. But his conviction for serious felonies involving the conduct of his office made his withdrawal from the Senate necessary and inevitable. The Senate Ethics Committee gave him a full and fair hearing and unanimously recommended expulsion. His colleagues listened to his case with courtesy and attention for six days. By yesterday, Sen. Williams saw what his fate would be and spared his friends the final burden of pronouncing judgment. It was the right thing to do.