WORKING ITS way through the federal bureaucracy is a petition asking President Carter to grant executive clemency to Patricia Hearst. When it reaches his desk, regardless of the recommendation that accompanies it from the Department of Justice, the president should grant it. Miss Hearst, who has served almost two years of a seven-year prison sentence imposed for a bank robbery in San Francisco, has served long enough.

The story of what has happened to Miss Hearst in the past five years is well known. It began in February 1974 when she was kidnapped by a group calling itself the Symbionese Liberation Army. Then came the million-dollar ransom paid by her family, her announcement that she was staying with the SLA, her participation in the bank robberty and the robbery of a Los Angeles sporting-goods store, her trial, in which she claimed to have been coerced into committing those crimes and, finally, her conviction in September 1976.

The campaign to persuade President Carter to pardon her or reduce her sentence is not so well known, although it has attracted a remarkably diverse group of supporters. They include members of the jury that convicted her, the FBI agent in charge of the case at the time she was arrested, dozens of members of Congress and many newspapers across the country. The campaign has two themes. One is that Miss Hearst was "brainwashed" by the SLA and, therefore, should not have been punished for what she did. The other theme is that, even if Miss Hearst had not been brainwashed, she has been sufficiently punished for her crimes.

There is merit to both arguments, but to us, the most persuasive argument on Miss Hearst's behalf was made by the prosecutor when she was about to be sentenced for the Los Angeles robbery. The crime in that case was more serious than the bank robbery in San Francisco. She admitted spraying the store with machine-gun bullets to cover the escape of her comrades. It was a miracle no one was killed. But after she pleaded guilty to crimes for which she could have received life imprisonment, the prosecution recommended probation. "I do not believe Miss Hearst presents any threat to the community any longer," Deputy District Attorney Sam Mayerson told the judge. The judge accepted the recommendation, took note of what he called "the 57 days of horrible torture" she had suffered after she was kidnapped, and placed her on probation for five years.

In considering the clemency petition, President Carter does not have to choose between these different themes. All lead to the same conclusion. No useful purpose can be served by keeping Miss Hearst in prison even for the six months that remain before she becomes eligible for parole from the bank-robbery sentence. She has paid her debt to society and is entitled to begin to pick up the pieces of a life that -- regardless of what she did or said afterward -- was shattered when she was kidnapped. Whatever the purposes of imprisonment are -- to deter other from committing crime, to protect the community, to rehabilitate the criminal or to exact retribution -- they have been met in this case. Miss Hearst should go free.